diff --git a/boa/src/builtins/object/internal_methods_trait.rs b/boa/src/builtins/object/internal_methods_trait.rs
index 5a03cb5b72..6d8a2a5a5e 100644
--- a/boa/src/builtins/object/internal_methods_trait.rs
+++ b/boa/src/builtins/object/internal_methods_trait.rs
@@ -6,7 +6,7 @@
 //! [spec]: https://tc39.es/ecma262/#sec-ordinary-object-internal-methods-and-internal-slots
 
 use crate::builtins::{
-    object::{Object, PROTOTYPE},
+    object::{Object, INSTANCE_PROTOTYPE},
     property::Property,
     value::{to_value, Value, ValueData},
 };
@@ -178,7 +178,7 @@ pub trait ObjectInternalMethods {
     /// Returns either the prototype or null
     /// https://tc39.es/ecma262/#sec-ordinary-object-internal-methods-and-internal-slots-getprototypeof
     fn get_prototype_of(&self) -> Value {
-        self.get_internal_slot(PROTOTYPE)
+        self.get_internal_slot(INSTANCE_PROTOTYPE)
     }
 
     fn define_own_property(&mut self, property_key: String, desc: Property) -> bool;
diff --git a/boa/src/builtins/value/mod.rs b/boa/src/builtins/value/mod.rs
index ff20598631..cb39ba0a77 100644
--- a/boa/src/builtins/value/mod.rs
+++ b/boa/src/builtins/value/mod.rs
@@ -13,7 +13,7 @@ use crate::builtins::{
     },
     property::Property,
 };
-use gc::{Gc, GcCell};
+use gc::{Gc, GcCell, GcCellRef};
 use gc_derive::{Finalize, Trace};
 use serde_json::{map::Map, Number as JSONNumber, Value as JSONValue};
 use std::{
@@ -238,6 +238,13 @@ impl ValueData {
         }
     }
 
+    pub fn as_object(&self) -> Option<GcCellRef<'_, Object>> {
+        match *self {
+            ValueData::Object(ref o) => Some(o.borrow()),
+            _ => None,
+        }
+    }
+
     /// remove_prop removes a property from a Value object.
     ///
     /// It will return a boolean based on if the value was removed, if there was no value to remove false is returned
diff --git a/boa/src/exec/mod.rs b/boa/src/exec/mod.rs
index ce889777b8..6b2487b856 100644
--- a/boa/src/exec/mod.rs
+++ b/boa/src/exec/mod.rs
@@ -11,6 +11,7 @@ use crate::{
             internal_methods_trait::ObjectInternalMethods, ObjectKind, INSTANCE_PROTOTYPE,
             PROTOTYPE,
         },
+        property::Property,
         value::{from_value, to_value, ResultValue, Value, ValueData},
     },
     environment::lexical_environment::{
@@ -347,6 +348,13 @@ impl Executor for Interpreter {
                     CompOp::GreaterThanOrEqual => v_a.to_num() >= v_b.to_num(),
                     CompOp::LessThan => v_a.to_num() < v_b.to_num(),
                     CompOp::LessThanOrEqual => v_a.to_num() <= v_b.to_num(),
+                    CompOp::In => {
+                        if !v_b.is_object() {
+                            panic!("TypeError: {} is not an Object.", v_b);
+                        }
+                        let key = self.to_property_key(v_a);
+                        self.has_property(v_b, &key)
+                    }
                 }))
             }
             Node::BinOp(BinOp::Log(ref op), ref a, ref b) => {
@@ -739,6 +747,31 @@ impl Interpreter {
         }
     }
 
+    /// The abstract operation ToPropertyKey takes argument argument. It converts argument to a value that can be used as a property key.
+    /// https://tc39.es/ecma262/#sec-topropertykey
+    #[allow(clippy::wrong_self_convention)]
+    pub fn to_property_key(&mut self, value: &Value) -> Value {
+        let key = self.to_primitive(value, Some("string"));
+        if key.is_symbol() {
+            key
+        } else {
+            self.to_string(&key)
+        }
+    }
+
+    /// https://tc39.es/ecma262/#sec-hasproperty
+    pub fn has_property(&self, obj: &Value, key: &Value) -> bool {
+        if let Some(obj) = obj.as_object() {
+            if !Property::is_property_key(key) {
+                false
+            } else {
+                obj.has_property(key)
+            }
+        } else {
+            false
+        }
+    }
+
     /// The abstract operation ToObject converts argument to a value of type Object
     /// https://tc39.es/ecma262/#sec-toobject
     #[allow(clippy::wrong_self_convention)]
diff --git a/boa/src/exec/tests.rs b/boa/src/exec/tests.rs
index 0932e79500..720cf8c563 100644
--- a/boa/src/exec/tests.rs
+++ b/boa/src/exec/tests.rs
@@ -351,3 +351,72 @@ fn test_unary_post() {
     "#;
     assert_eq!(exec(execs_after), String::from("true"));
 }
+
+#[cfg(test)]
+mod in_operator {
+    use super::*;
+    #[test]
+    fn test_p_in_o() {
+        let p_in_o = r#"
+            var o = {a: 'a'};
+            var p = 'a';
+            p in o
+        "#;
+        assert_eq!(exec(p_in_o), String::from("true"));
+    }
+
+    #[test]
+    fn test_p_in_property_chain() {
+        let p_in_o = r#"
+            var o = {};
+            var p = 'toString';
+            p in o
+        "#;
+        assert_eq!(exec(p_in_o), String::from("true"));
+    }
+
+    #[test]
+    fn test_p_not_in_o() {
+        let p_not_in_o = r#"
+            var o = {a: 'a'};
+            var p = 'b';
+            p in o
+        "#;
+        assert_eq!(exec(p_not_in_o), String::from("false"));
+    }
+
+    #[test]
+    fn test_number_in_array() {
+        // Note: this is valid because the LHS is converted to a prop key with ToPropertyKey
+        // and arrays are just fancy objects like {'0': 'a'}
+        let num_in_array = r#"
+            var n = 0;
+            var a = ['a'];
+            n in a
+        "#;
+        assert_eq!(exec(num_in_array), String::from("true"));
+    }
+
+    #[test]
+    #[ignore]
+    fn test_symbol_in_object() {
+        // FIXME: this scenario works in Firefox's console, this is probably an issue
+        // with Symbol comparison.
+        let sym_in_object = r#"
+            var sym = Symbol('hi');
+            var o = {};
+            o[sym] = 'hello';
+            sym in o
+        "#;
+        assert_eq!(exec(sym_in_object), String::from("true"));
+    }
+
+    #[test]
+    #[should_panic(expected = "TypeError: undefined is not an Object.")]
+    fn test_should_type_error_when_rhs_not_object() {
+        let scenario = r#"
+            'fail' in undefined
+        "#;
+        exec(scenario);
+    }
+}
diff --git a/boa/src/syntax/ast/keyword.rs b/boa/src/syntax/ast/keyword.rs
index 2ef9599cbc..a76195e5a8 100644
--- a/boa/src/syntax/ast/keyword.rs
+++ b/boa/src/syntax/ast/keyword.rs
@@ -7,7 +7,9 @@
 //! [spec]: https://www.ecma-international.org/ecma-262/#sec-keywords
 //! [mdn]: https://developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/Lexical_grammar#Keywords
 
+use crate::syntax::ast::op::{BinOp, CompOp};
 use std::{
+    convert::TryInto,
     error,
     fmt::{Display, Error, Formatter},
     str::FromStr,
@@ -433,6 +435,23 @@ pub enum Keyword {
     Yield,
 }
 
+impl Keyword {
+    pub fn as_binop(self) -> Option<BinOp> {
+        match self {
+            Keyword::In => Some(BinOp::Comp(CompOp::In)),
+            _ => None,
+        }
+    }
+}
+
+impl TryInto<BinOp> for Keyword {
+    type Error = String;
+    fn try_into(self) -> Result<BinOp, Self::Error> {
+        self.as_binop()
+            .ok_or_else(|| format!("No binary operation for {}", self))
+    }
+}
+
 #[derive(Debug, Clone, Copy)]
 pub struct KeywordError;
 impl Display for KeywordError {
diff --git a/boa/src/syntax/ast/op.rs b/boa/src/syntax/ast/op.rs
index 34552aab4f..f8a67cffbb 100644
--- a/boa/src/syntax/ast/op.rs
+++ b/boa/src/syntax/ast/op.rs
@@ -579,6 +579,19 @@ pub enum CompOp {
     /// [spec]: https://tc39.es/ecma262/#prod-RelationalExpression
     /// [mdn]: https://developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/Operators/Comparison_Operators#Less_than_or_equal_operator
     LessThanOrEqual,
+    /// The `in` operator returns true if the specified property is in the specified object or its prototype chain.
+    ///
+    /// Syntax: `prop in object`
+    ///
+    /// Returns `true` the specified property is in the specified object or its prototype chain.
+    ///
+    /// More information:
+    ///  - [ECMAScript reference][spec]
+    ///  - [MDN documentation][mdn]
+    ///
+    /// [spec]: https://tc39.es/ecma262/#prod-RelationalExpression
+    /// [mdn]: https://developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/Operators/in
+    In,
 }
 
 impl Display for CompOp {
@@ -595,6 +608,7 @@ impl Display for CompOp {
                 Self::GreaterThanOrEqual => ">=",
                 Self::LessThan => "<",
                 Self::LessThanOrEqual => "<=",
+                Self::In => "in",
             }
         )
     }
@@ -727,7 +741,8 @@ impl Operator for BinOp {
             Self::Comp(CompOp::LessThan)
             | Self::Comp(CompOp::LessThanOrEqual)
             | Self::Comp(CompOp::GreaterThan)
-            | Self::Comp(CompOp::GreaterThanOrEqual) => 8,
+            | Self::Comp(CompOp::GreaterThanOrEqual)
+            | Self::Comp(CompOp::In) => 8,
             Self::Comp(CompOp::Equal)
             | Self::Comp(CompOp::NotEqual)
             | Self::Comp(CompOp::StrictEqual)
diff --git a/boa/src/syntax/ast/punc.rs b/boa/src/syntax/ast/punc.rs
index 6b237e3305..dab0dfcf70 100644
--- a/boa/src/syntax/ast/punc.rs
+++ b/boa/src/syntax/ast/punc.rs
@@ -6,7 +6,10 @@
 //! [spec]: https://tc39.es/ecma262/#prod-Punctuator
 
 use crate::syntax::ast::op::{AssignOp, BinOp, BitOp, CompOp, LogOp, NumOp};
-use std::fmt::{Display, Error, Formatter};
+use std::{
+    convert::TryInto,
+    fmt::{Display, Error, Formatter},
+};
 
 #[cfg(feature = "serde")]
 use serde::{Deserialize, Serialize};
@@ -169,6 +172,14 @@ impl Punctuator {
     }
 }
 
+impl TryInto<BinOp> for Punctuator {
+    type Error = String;
+    fn try_into(self) -> Result<BinOp, Self::Error> {
+        self.as_binop()
+            .ok_or_else(|| format!("No binary operation for {}", self))
+    }
+}
+
 impl Display for Punctuator {
     fn fmt(&self, f: &mut Formatter<'_>) -> Result<(), Error> {
         write!(
diff --git a/boa/src/syntax/parser/expression/mod.rs b/boa/src/syntax/parser/expression/mod.rs
index d225fe9d45..464784449b 100644
--- a/boa/src/syntax/parser/expression/mod.rs
+++ b/boa/src/syntax/parser/expression/mod.rs
@@ -18,7 +18,23 @@ mod update;
 use self::assignment::ExponentiationExpression;
 pub(super) use self::{assignment::AssignmentExpression, primary::Initializer};
 use super::{AllowAwait, AllowIn, AllowYield, Cursor, ParseResult, TokenParser};
-use crate::syntax::ast::{node::Node, punc::Punctuator, token::TokenKind};
+use crate::syntax::ast::{keyword::Keyword, node::Node, punc::Punctuator, token::TokenKind};
+
+// For use in the expression! macro to allow for both Punctuator and Keyword parameters.
+// Always returns false.
+impl PartialEq<Keyword> for Punctuator {
+    fn eq(&self, _other: &Keyword) -> bool {
+        false
+    }
+}
+
+// For use in the expression! macro to allow for both Punctuator and Keyword parameters.
+// Always returns false.
+impl PartialEq<Punctuator> for Keyword {
+    fn eq(&self, _other: &Punctuator) -> bool {
+        false
+    }
+}
 
 /// Generates an expression parser.
 ///
@@ -38,7 +54,15 @@ macro_rules! expression { ($name:ident, $lower:ident, [$( $op:path ),*], [$( $lo
                     TokenKind::Punctuator(op) if $( op == $op )||* => {
                         let _ = cursor.next().expect("token disappeared");
                         lhs = Node::bin_op(
-                            op.as_binop().expect("could not get binary operation"),
+                            op.as_binop().expect("Could not get binary operation."),
+                            lhs,
+                            $lower::new($( self.$low_param ),*).parse(cursor)?
+                        )
+                    }
+                    TokenKind::Keyword(op) if $( op == $op )||* => {
+                        let _ = cursor.next().expect("token disappeared");
+                        lhs = Node::bin_op(
+                            op.as_binop().expect("Could not get binary operation."),
                             lhs,
                             $lower::new($( self.$low_param ),*).parse(cursor)?
                         )
@@ -360,7 +384,8 @@ expression!(
         Punctuator::LessThan,
         Punctuator::GreaterThan,
         Punctuator::LessThanOrEq,
-        Punctuator::GreaterThanOrEq
+        Punctuator::GreaterThanOrEq,
+        Keyword::In
     ],
     [allow_yield, allow_await]
 );
diff --git a/boa/src/syntax/parser/expression/tests.rs b/boa/src/syntax/parser/expression/tests.rs
index 3b104ec885..11c21ee8e6 100644
--- a/boa/src/syntax/parser/expression/tests.rs
+++ b/boa/src/syntax/parser/expression/tests.rs
@@ -1,6 +1,6 @@
 use crate::syntax::{
     ast::node::Node,
-    ast::op::{AssignOp, BinOp, BitOp, NumOp},
+    ast::op::{AssignOp, BinOp, BitOp, CompOp, NumOp},
     parser::tests::check_parser,
 };
 
@@ -291,3 +291,47 @@ fn check_assign_operations() {
         )],
     );
 }
+
+#[test]
+fn check_relational_operations() {
+    check_parser(
+        "a < b",
+        &[Node::bin_op(
+            BinOp::Comp(CompOp::LessThan),
+            Node::Local(String::from("a")),
+            Node::Local(String::from("b")),
+        )],
+    );
+    check_parser(
+        "a > b",
+        &[Node::bin_op(
+            BinOp::Comp(CompOp::GreaterThan),
+            Node::Local(String::from("a")),
+            Node::Local(String::from("b")),
+        )],
+    );
+    check_parser(
+        "a <= b",
+        &[Node::bin_op(
+            BinOp::Comp(CompOp::LessThanOrEqual),
+            Node::Local(String::from("a")),
+            Node::Local(String::from("b")),
+        )],
+    );
+    check_parser(
+        "a >= b",
+        &[Node::bin_op(
+            BinOp::Comp(CompOp::GreaterThanOrEqual),
+            Node::Local(String::from("a")),
+            Node::Local(String::from("b")),
+        )],
+    );
+    check_parser(
+        "p in o",
+        &[Node::bin_op(
+            BinOp::Comp(CompOp::In),
+            Node::Local(String::from("p")),
+            Node::Local(String::from("o")),
+        )],
+    );
+}
diff --git a/boa/src/syntax/parser/tests.rs b/boa/src/syntax/parser/tests.rs
index 088f63ebd8..cbc38564cc 100644
--- a/boa/src/syntax/parser/tests.rs
+++ b/boa/src/syntax/parser/tests.rs
@@ -39,7 +39,7 @@ fn check_construct_call_precedence() {
 }
 
 #[test]
-fn assing_operator_precedence() {
+fn assign_operator_precedence() {
     check_parser(
         "a = a + 1",
         &[Node::assign(