diff --git a/interpreter/operator_arithmetic.go b/interpreter/operator_arithmetic.go
index f293536..7e8801b 100644
--- a/interpreter/operator_arithmetic.go
+++ b/interpreter/operator_arithmetic.go
@@ -132,6 +132,22 @@ func evalFloor(_ model.IUnaryExpression, obj result.Value) (result.Value, error)
 	return result.New(int32(math.Floor(val)))
 }
 
+// Ln(argument Decimal) Decimal
+// https://cql.hl7.org/09-b-cqlreference.html#ln
+func evalLn(_ model.IUnaryExpression, obj result.Value) (result.Value, error) {
+	if result.IsNull(obj) {
+		return result.New(nil)
+	}
+	val, err := result.ToFloat64(obj)
+	if err != nil {
+		return result.Value{}, err
+	}
+	if val <= 0 {
+		return result.New(nil)
+	}
+	return result.New(math.Log(val))
+}
+
 // op(left Integer, right Integer) Integer
 // https://cql.hl7.org/09-b-cqlreference.html#add
 // https://cql.hl7.org/09-b-cqlreference.html#subtract
diff --git a/interpreter/operator_dispatcher.go b/interpreter/operator_dispatcher.go
index 32b94ab..5b098f1 100644
--- a/interpreter/operator_dispatcher.go
+++ b/interpreter/operator_dispatcher.go
@@ -150,6 +150,13 @@ func (i *interpreter) unaryOverloads(m model.IUnaryExpression) ([]convert.Overlo
 				Result:   evalFloor,
 			},
 		}, nil
+	case *model.Ln:
+		return []convert.Overload[evalUnarySignature]{
+			{
+				Operands: []types.IType{types.Decimal},
+				Result:   evalLn,
+			},
+		}, nil
 	case *model.Exists:
 		return []convert.Overload[evalUnarySignature]{
 			{
diff --git a/model/model.go b/model/model.go
index 1d559b8..a4c26fe 100644
--- a/model/model.go
+++ b/model/model.go
@@ -660,6 +660,9 @@ type Ceiling struct{ *UnaryExpression }
 // Floor  is https://cql.hl7.org/04-logicalspecification.html#floor.
 type Floor struct{ *UnaryExpression }
 
+// Ln  is https://cql.hl7.org/04-logicalspecification.html#ln.
+type Ln struct{ *UnaryExpression }
+
 // SingletonFrom is https://cql.hl7.org/04-logicalspecification.html#singletonfrom.
 type SingletonFrom struct{ *UnaryExpression }
 
@@ -1129,6 +1132,9 @@ func (a *Ceiling) GetName() string { return "Ceiling" }
 // GetName returns the name of the system operator.
 func (a *Floor) GetName() string { return "Floor" }
 
+// GetName returns the name of the system operator.
+func (a *Ln) GetName() string { return "Ln" }
+
 // GetName returns the name of the system operator.
 func (a *As) GetName() string { return "As" }
 
diff --git a/parser/operators.go b/parser/operators.go
index 46b7154..e2a8f13 100644
--- a/parser/operators.go
+++ b/parser/operators.go
@@ -666,6 +666,17 @@ func (p *Parser) loadSystemOperators() error {
 				}
 			},
 		},
+		{
+			name:     "Ln",
+			operands: [][]types.IType{{types.Decimal}},
+			model: func() model.IExpression {
+				return &model.Ln{
+					UnaryExpression: &model.UnaryExpression{
+						Expression: model.ResultType(types.Decimal),
+					},
+				}
+			},
+		},
 		{
 			name:     "Negate",
 			operands: [][]types.IType{{types.Integer}},
diff --git a/parser/operators_test.go b/parser/operators_test.go
index fe5d548..3b51ec9 100644
--- a/parser/operators_test.go
+++ b/parser/operators_test.go
@@ -464,6 +464,16 @@ func TestBuiltInFunctions(t *testing.T) {
 				},
 			},
 		},
+		{
+			name: "Arithmetic Ln",
+			cql:  "Ln(1.0)",
+			want: &model.Ln{
+				UnaryExpression: &model.UnaryExpression{
+					Operand:    model.NewLiteral("1.0", types.Decimal),
+					Expression: model.ResultType(types.Decimal),
+				},
+			},
+		},
 		{
 			name: "Arithmetic Addition",
 			cql:  "Add(1, 2)",
diff --git a/tests/enginetests/operator_arithmetic_test.go b/tests/enginetests/operator_arithmetic_test.go
index 3a86472..f048453 100644
--- a/tests/enginetests/operator_arithmetic_test.go
+++ b/tests/enginetests/operator_arithmetic_test.go
@@ -344,6 +344,84 @@ func TestFloor(t *testing.T) {
 	}
 }
 
+func TestLn(t *testing.T) {
+	tests := []struct {
+		name       string
+		cql        string
+		wantModel  model.IExpression
+		wantResult result.Value
+	}{
+		{
+			name: "Decimal",
+			cql:  "Ln(1.0)",
+			wantModel: &model.Ln{
+				UnaryExpression: &model.UnaryExpression{
+					Operand:    model.NewLiteral("1.0", types.Decimal),
+					Expression: model.ResultType(types.Decimal),
+				},
+			},
+			wantResult: newOrFatal(t, 0.0),
+		},
+		{
+			name:       "Negative",
+			cql:        "Ln(-2.1)",
+			wantResult: newOrFatal(t, nil),
+		},
+		{
+			name:       "Zero",
+			cql:        "Ln(0.0)",
+			wantResult: newOrFatal(t, nil),
+		},
+		{
+			name:       "Ten",
+			cql:        "Ln(10.0)",
+			wantResult: newOrFatal(t, 2.302585092994046),
+		},
+		{
+			name:       "Integer",
+			cql:        "Ln(1)",
+			wantResult: newOrFatal(t, 0.0),
+		},
+		{
+			name:       "Minimum Integer",
+			cql:        "Ln(-2147483648)",
+			wantResult: newOrFatal(t, nil),
+		},
+		{
+			name:       "Maximum Integer",
+			cql:        "Ln(2147483647.0)",
+			wantResult: newOrFatal(t, 21.487562596892644),
+		},
+		{
+			name:       "Null",
+			cql:        "Ln(null as Decimal)",
+			wantResult: newOrFatal(t, nil),
+		},
+	}
+
+	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)
+			}
+
+			if diff := cmp.Diff(tc.wantResult, getTESTRESULT(t, results), protocmp.Transform()); diff != "" {
+				t.Errorf("Eval diff (-want +got)\n%v", diff)
+			}
+		})
+	}
+}
+
 func TestAdd(t *testing.T) {
 	tests := []struct {
 		name       string
diff --git a/tests/spectests/exclusions/exclusions.go b/tests/spectests/exclusions/exclusions.go
index f7db870..7a1ca35 100644
--- a/tests/spectests/exclusions/exclusions.go
+++ b/tests/spectests/exclusions/exclusions.go
@@ -55,7 +55,6 @@ func XMLTestFileExclusionDefinitions() map[string]XMLTestFileExclusions {
 				"HighBoundary",
 				"Log",
 				"LowBoundary",
-				"Ln",
 				"Precision",
 				"Round",
 			},
@@ -64,6 +63,8 @@ func XMLTestFileExclusionDefinitions() map[string]XMLTestFileExclusions {
 				"Divide103",
 				"Multiply1CMBy2CM",
 				"Power2DToNeg2DEquivalence",
+				"Ln1000",  // Require Round support.
+				"Ln1000D", // Require Round support.
 				// TODO: b/342061606 - Unit conversion is not supported.
 				"Divide1Q1",
 				"Divide10Q5I",