diff --git a/interpreter/operator_dispatcher.go b/interpreter/operator_dispatcher.go
index 160c64a..3c0bb9c 100644
--- a/interpreter/operator_dispatcher.go
+++ b/interpreter/operator_dispatcher.go
@@ -936,6 +936,13 @@ func (i *interpreter) binaryOverloads(m model.IBinaryExpression) ([]convert.Over
 				Result:   i.evalIndexerList,
 			},
 		}, nil
+	case *model.IndexOf:
+		return []convert.Overload[evalBinarySignature]{
+			{
+				Operands: []types.IType{&types.List{ElementType: types.Any}, types.Any},
+				Result:   evalIndexOf,
+			},
+		}, nil
 	default:
 		return nil, fmt.Errorf("unsupported Binary Expression %v", m.GetName())
 	}
diff --git a/interpreter/operator_list.go b/interpreter/operator_list.go
index 253935a..3c1c7c5 100644
--- a/interpreter/operator_list.go
+++ b/interpreter/operator_list.go
@@ -120,6 +120,28 @@ func evalLast(m model.IUnaryExpression, listObj result.Value) (result.Value, err
 	return list[len(list)-1], nil
 }
 
+// IndexOf(argument List<T>, element T) Integer
+// https://cql.hl7.org/09-b-cqlreference.html#indexof
+func evalIndexOf(m model.IBinaryExpression, listObj, valueObj result.Value) (result.Value, error) {
+	if result.IsNull(listObj) || result.IsNull(valueObj) {
+		return result.New(nil)
+	}
+	list, err := result.ToSlice(listObj)
+	if err != nil {
+		return result.Value{}, err
+	}
+	if len(list) == 0 {
+		return result.New(int32(-1))
+	}
+
+	for i, elemObj := range list {
+		if valueObj.Equal(elemObj) {
+			return result.New(int32(i))
+		}
+	}
+	return result.New(int32(-1))
+}
+
 // singleton from(argument List<T>) T
 // https://cql.hl7.org/09-b-cqlreference.html#singleton-from
 func evalSingletonFrom(m model.IUnaryExpression, listObj result.Value) (result.Value, error) {
diff --git a/model/model.go b/model/model.go
index 9e25ffc..82933a3 100644
--- a/model/model.go
+++ b/model/model.go
@@ -975,6 +975,11 @@ type Split struct{ *BinaryExpression }
 // Indexer ELM Expression https://cql.hl7.org/04-logicalspecification.html#indexer.
 type Indexer struct{ *BinaryExpression }
 
+// IndexOf ELM Expression https://cql.hl7.org/04-logicalspecification.html#indexof.
+// IndexOf is an OperatorExpression in ELM, but we're modeling it as a BinaryExpression since in CQL
+// it always takes two arguments.
+type IndexOf struct{ *BinaryExpression }
+
 // BinaryExpressionWithPrecision represents a BinaryExpression with a precision property.
 type BinaryExpressionWithPrecision struct {
 	*BinaryExpression
@@ -1453,6 +1458,9 @@ func (a *Combine) GetName() string { return "Combine" }
 // GetName returns the name of the system operator.
 func (i *Indexer) GetName() string { return "Indexer" }
 
+// GetName returns the name of the system operator.
+func (a *IndexOf) GetName() string { return "IndexOf" }
+
 // GetName returns the name of the system operator.
 func (m *Median) GetName() string { return "Median" }
 
diff --git a/parser/operators.go b/parser/operators.go
index d4f4e8a..c9a19bd 100644
--- a/parser/operators.go
+++ b/parser/operators.go
@@ -1587,6 +1587,19 @@ func (p *Parser) loadSystemOperators() error {
 				}
 			},
 		},
+		{
+			name: "IndexOf",
+			operands: [][]types.IType{
+				{&types.List{ElementType: types.Any}, types.Any},
+			},
+			model: func() model.IExpression {
+				return &model.IndexOf{
+					BinaryExpression: &model.BinaryExpression{
+						Expression: model.ResultType(types.Integer),
+					},
+				}
+			},
+		},
 		{
 			name: "Last",
 			operands: [][]types.IType{
diff --git a/parser/operators_test.go b/parser/operators_test.go
index 0c904ab..c62915f 100644
--- a/parser/operators_test.go
+++ b/parser/operators_test.go
@@ -1238,6 +1238,19 @@ func TestBuiltInFunctions(t *testing.T) {
 				},
 			},
 		},
+		{
+			name: "IndexOf",
+			cql:  "IndexOf({1, 2, 3}, 1)",
+			want: &model.IndexOf{
+				BinaryExpression: &model.BinaryExpression{
+					Operands: []model.IExpression{
+						model.NewList([]string{"1", "2", "3"}, types.Integer),
+						model.NewLiteral("1", types.Integer),
+					},
+					Expression: model.ResultType(types.Integer),
+				},
+			},
+		},
 		{
 			name: "Last",
 			cql:  "Last({1})",
diff --git a/tests/enginetests/operator_list_test.go b/tests/enginetests/operator_list_test.go
index 3d1b71c..3bec281 100644
--- a/tests/enginetests/operator_list_test.go
+++ b/tests/enginetests/operator_list_test.go
@@ -433,6 +433,84 @@ func TestLast(t *testing.T) {
 	}
 }
 
+func TestIndexOf(t *testing.T) {
+	tests := []struct {
+		name                 string
+		cql                  string
+		wantModel            model.IExpression
+		wantResult           result.Value
+		wantSourceExpression model.IExpression
+		wantSourceValues     []result.Value
+	}{
+		{
+			name: "IndexOf({1, 2}, 2) = 1",
+			cql:  "IndexOf({1, 2}, 2)",
+			wantModel: &model.IndexOf{
+				BinaryExpression: &model.BinaryExpression{
+					Operands: []model.IExpression{
+						model.NewList([]string{"1", "2"}, types.Integer),
+						model.NewLiteral("2", types.Integer),
+					},
+					Expression: model.ResultType(types.Integer),
+				},
+			},
+			wantResult: newOrFatal(t, int32(1)),
+		},
+		{
+			name:       "IndexOf(List<Integer>{}, 1) = -1",
+			cql:        "IndexOf(List<Integer>{}, 1)",
+			wantResult: newOrFatal(t, -1),
+		},
+		{
+			name:       "IndexOf(null as List<Integer>, 1) = null",
+			cql:        "IndexOf(null as List<Integer>, 1)",
+			wantResult: newOrFatal(t, nil),
+		},
+		{
+			name:       "IndexOf({1, 2}, null as Integer) = null",
+			cql:        "IndexOf({1, 2}, null as Integer)",
+			wantResult: newOrFatal(t, nil),
+		},
+		{
+			name:       "IndexOf({1, 2}, 3) = -1",
+			cql:        "IndexOf({1, 2}, 3)",
+			wantResult: newOrFatal(t, -1),
+		},
+		{
+			name:       "IndexOf({@2010, @2011, @2012}, @2011) = 1",
+			cql:        "IndexOf({@2010, @2011, @2012}, @2011)",
+			wantResult: newOrFatal(t, 1),
+		},
+	}
+	for _, tc := range tests {
+		t.Run(tc.name, func(t *testing.T) {
+			p := newFHIRParser(t)
+			parsedLibs, err := p.Libraries(context.Background(), wrapInLib(t, tc.cql), parser.Config{})
+			if err != nil {
+				t.Fatalf("Parse returned unexpected error: %v", err)
+			}
+			if diff := cmp.Diff(tc.wantModel, getTESTRESULTModel(t, parsedLibs)); tc.wantModel != nil && diff != "" {
+				t.Errorf("Parse diff (-want +got):\n%s", diff)
+			}
+
+			results, err := interpreter.Eval(context.Background(), parsedLibs, defaultInterpreterConfig(t, p))
+			if err != nil {
+				t.Fatalf("Eval returned unexpected error: %v", err)
+			}
+			gotResult := getTESTRESULTWithSources(t, results)
+			if diff := cmp.Diff(tc.wantResult, gotResult, protocmp.Transform()); diff != "" {
+				t.Errorf("Eval returned diff (-want +got)\n%v", diff)
+			}
+			if diff := cmp.Diff(tc.wantSourceExpression, gotResult.SourceExpression(), protocmp.Transform()); tc.wantSourceExpression != nil && diff != "" {
+				t.Errorf("Eval SourceExpression diff (-want +got)\n%v", diff)
+			}
+			if diff := cmp.Diff(tc.wantSourceValues, gotResult.SourceValues(), protocmp.Transform()); tc.wantSourceValues != nil && diff != "" {
+				t.Errorf("Eval SourceValues diff (-want +got)\n%v", diff)
+			}
+		})
+	}
+}
+
 func TestSingletonFrom(t *testing.T) {
 	tests := []struct {
 		name       string
diff --git a/tests/spectests/exclusions/exclusions.go b/tests/spectests/exclusions/exclusions.go
index 84153d7..9e7e0ad 100644
--- a/tests/spectests/exclusions/exclusions.go
+++ b/tests/spectests/exclusions/exclusions.go
@@ -344,7 +344,6 @@ func XMLTestFileExclusionDefinitions() map[string]XMLTestFileExclusions {
 				"Flatten",
 				"Includes",
 				"IncludedIn",
-				"IndexOf",
 				"Intersect",
 				"Length",
 				"ProperContains",