diff --git a/internal/datehelpers/string.go b/internal/datehelpers/string.go
index 3ddbda2..da8e209 100644
--- a/internal/datehelpers/string.go
+++ b/internal/datehelpers/string.go
@@ -38,15 +38,18 @@ func DateString(d time.Time, precision model.DateTimePrecision) (string, error)
 }
 
 // DateTimeString returns a CQL DateTime string representation of a DateTime.
+// For Year, Month, and Day precision, we remove the 'T' value but still include the timezone.
+// The correct behavior here is unclear in the spec but this seems like the correct behavior for
+// now.
 func DateTimeString(d time.Time, precision model.DateTimePrecision) (string, error) {
 	var dtFormat string
 	switch precision {
 	case model.YEAR:
-		dtFormat = dateTimeYear
+		dtFormat = dateYear
 	case model.MONTH:
-		dtFormat = dateTimeMonth
+		dtFormat = dateMonth
 	case model.DAY:
-		dtFormat = dateTimeDay
+		dtFormat = dateDay
 	case model.HOUR:
 		dtFormat = dateTimeHour
 	case model.MINUTE:
diff --git a/interpreter/operator_dispatcher.go b/interpreter/operator_dispatcher.go
index 804bd82..966de5b 100644
--- a/interpreter/operator_dispatcher.go
+++ b/interpreter/operator_dispatcher.go
@@ -243,6 +243,49 @@ func (i *interpreter) unaryOverloads(m model.IUnaryExpression) ([]convert.Overlo
 				Result:   evalToConceptList,
 			},
 		}, nil
+	case *model.ToString:
+		return []convert.Overload[evalUnarySignature]{
+			{
+				Operands: []types.IType{types.Any},
+				Result:   evalToString,
+			},
+			{
+				Operands: []types.IType{types.Boolean},
+				Result:   evalToString,
+			},
+			{
+				Operands: []types.IType{types.Integer},
+				Result:   evalToString,
+			},
+			{
+				Operands: []types.IType{types.Long},
+				Result:   evalToString,
+			},
+			{
+				Operands: []types.IType{types.Decimal},
+				Result:   evalToString,
+			},
+			{
+				Operands: []types.IType{types.Quantity},
+				Result:   evalToString,
+			},
+			{
+				Operands: []types.IType{types.Ratio},
+				Result:   evalToString,
+			},
+			{
+				Operands: []types.IType{types.Date},
+				Result:   evalToString,
+			},
+			{
+				Operands: []types.IType{types.DateTime},
+				Result:   evalToString,
+			},
+			{
+				Operands: []types.IType{types.Time},
+				Result:   evalToString,
+			},
+		}, nil
 	case *model.End:
 		return []convert.Overload[evalUnarySignature]{
 			{
diff --git a/interpreter/operator_string.go b/interpreter/operator_string.go
index 224f9ef..b0b9e06 100644
--- a/interpreter/operator_string.go
+++ b/interpreter/operator_string.go
@@ -15,10 +15,14 @@
 package interpreter
 
 import (
+	"fmt"
+	"strconv"
 	"strings"
 
+	"github.com/google/cql/internal/datehelpers"
 	"github.com/google/cql/model"
 	"github.com/google/cql/result"
+	"github.com/google/cql/types"
 )
 
 // +(left String, right String) String
@@ -41,3 +45,101 @@ func evalConcatenate(m model.INaryExpression, operands []result.Value) (result.V
 	}
 	return result.New(retStr.String())
 }
+
+// ToString(Boolean) String
+// ToString(Integer) String
+// ToString(Long) String
+// ToString(Decimal) String
+// ToString(Quantity) String
+// ToString(Ratio) String
+// ToString(Date) String
+// ToString(DateTime) String
+// ToString(Time) String
+// https://cql.hl7.org/09-b-cqlreference.html#tostring
+// In the future we may put this logic directly onto the result.Value interface. Could be useful
+// for debugging, and/or the REPL.
+func evalToString(_ model.IUnaryExpression, operand result.Value) (result.Value, error) {
+	if result.IsNull(operand) {
+		return result.New(nil)
+	}
+
+	switch operand.RuntimeType() {
+	case types.Boolean:
+		b, err := result.ToBool(operand)
+		if err != nil {
+			return result.Value{}, err
+		}
+		return result.New(strconv.FormatBool(b))
+	case types.Integer:
+		i, err := result.ToInt32(operand)
+		if err != nil {
+			return result.Value{}, err
+		}
+		return result.New(strconv.FormatInt(int64(i), 10))
+	case types.Long:
+		i, err := result.ToInt64(operand)
+		if err != nil {
+			return result.Value{}, err
+		}
+		return result.New(strconv.FormatInt(i, 10))
+	case types.Decimal:
+		d, err := result.ToFloat64(operand)
+		if err != nil {
+			return result.Value{}, err
+		}
+		return result.New(strconv.FormatFloat(d, 'f', -1, 64))
+	case types.Quantity:
+		q, err := result.ToQuantity(operand)
+		if err != nil {
+			return result.Value{}, err
+		}
+		return result.New(quantityToString(q))
+	case types.Ratio:
+		r, err := result.ToRatio(operand)
+		if err != nil {
+			return result.Value{}, err
+		}
+		return result.New(fmt.Sprintf("%s:%s", quantityToString(r.Numerator), quantityToString(r.Denominator)))
+	case types.Date:
+		d, err := result.ToDateTime(operand)
+		if err != nil {
+			return result.Value{}, err
+		}
+		s, err := datehelpers.DateString(d.Date, d.Precision)
+		if err != nil {
+			return result.Value{}, err
+		}
+		// Remove the leading '@'
+		return result.New(s[1:])
+	case types.DateTime:
+		d, err := result.ToDateTime(operand)
+		if err != nil {
+			return result.Value{}, err
+		}
+		s, err := datehelpers.DateTimeString(d.Date, d.Precision)
+		if err != nil {
+			return result.Value{}, err
+		}
+		// Remove the leading '@'
+		return result.New(s[1:])
+	case types.Time:
+		t, err := result.ToDateTime(operand)
+		if err != nil {
+			return result.Value{}, err
+		}
+		s, err := datehelpers.TimeString(t.Date, t.Precision)
+		if err != nil {
+			return result.Value{}, err
+		}
+		// Remove the leading 'T'
+		return result.New(s[1:])
+	default:
+		return result.Value{}, fmt.Errorf("unsupported operand type for ToString: %v", operand.RuntimeType())
+	}
+}
+
+// convert a quantity value to a string
+func quantityToString(q result.Quantity) string {
+	f := strconv.FormatFloat(q.Value, 'f', -1, 64)
+	return fmt.Sprintf("%s '%s'", f, q.Unit)
+}
diff --git a/parser/operators.go b/parser/operators.go
index a4820dd..da5bd68 100644
--- a/parser/operators.go
+++ b/parser/operators.go
@@ -395,6 +395,7 @@ func (p *Parser) loadSystemOperators() error {
 		{
 			name: "ToString",
 			operands: [][]types.IType{
+				{types.Any},
 				{types.String},
 				{types.Integer},
 				{types.Long},
diff --git a/tests/enginetests/operator_string_test.go b/tests/enginetests/operator_string_test.go
index 655ab72..aa0abf8 100644
--- a/tests/enginetests/operator_string_test.go
+++ b/tests/enginetests/operator_string_test.go
@@ -119,3 +119,118 @@ func TestConcatenate(t *testing.T) {
 		})
 	}
 }
+
+func TestToString(t *testing.T) {
+	tests := []struct {
+		name       string
+		cql        string
+		wantModel  model.IExpression
+		wantResult result.Value
+	}{
+		{
+			name:       "ToString(true)",
+			cql:        "ToString(true)",
+			wantResult: newOrFatal(t, "true"),
+		},
+		{
+			name:       "ToString(false)",
+			cql:        "ToString(false)",
+			wantResult: newOrFatal(t, "false"),
+		},
+		{
+			name:       "ToString(1)",
+			cql:        "ToString(1)",
+			wantResult: newOrFatal(t, "1"),
+		},
+		{
+			name:       "ToString(-1)",
+			cql:        "ToString(-1)",
+			wantResult: newOrFatal(t, "-1"),
+		},
+		{
+			name:       "ToString(100000L)",
+			cql:        "ToString(100000L)",
+			wantResult: newOrFatal(t, "100000"),
+		},
+		{
+			name:       "ToString(-100000L)",
+			cql:        "ToString(-100000L)",
+			wantResult: newOrFatal(t, "-100000"),
+		},
+		{
+			name:       "ToString(1.42)",
+			cql:        "ToString(1.42)",
+			wantResult: newOrFatal(t, "1.42"),
+		},
+		{
+			name:       "ToString(-1.42)",
+			cql:        "ToString(-1.42)",
+			wantResult: newOrFatal(t, "-1.42"),
+		},
+		{
+			name:       "ToString(1 'cm')",
+			cql:        "ToString(1 'cm')",
+			wantResult: newOrFatal(t, "1 'cm'"),
+		},
+		{
+			name:       "ToString(-1 'cm')",
+			cql:        "ToString(-1 'cm')",
+			wantResult: newOrFatal(t, "-1 'cm'"),
+		},
+		{
+			name:       "ToString(1'g':0.1'g')",
+			cql:        "ToString(1'g':0.1'g')",
+			wantResult: newOrFatal(t, "1 'g':0.1 'g'"),
+		},
+		{
+			name:       "ToString(@2022-01-03)",
+			cql:        "ToString(@2022-01-03)",
+			wantResult: newOrFatal(t, "2022-01-03"),
+		},
+		{
+			name:       "ToString(@2022-01-03T12:00:00Z)",
+			cql:        "ToString(@2022-01-03T12:00:00Z)",
+			wantResult: newOrFatal(t, "2022-01-03T12:00:00Z"),
+		},
+		{
+			name:       "ToString(DateTime(2022, 1, 3))",
+			cql:        "ToString(DateTime(2022, 1, 3))",
+			wantResult: newOrFatal(t, "2022-01-03+04:00"),
+		},
+		{
+			name:       "ToString(@T12:01:00)",
+			cql:        "ToString(@T12:01:00)",
+			wantResult: newOrFatal(t, "12:01:00"),
+		},
+		{
+			name:       "ToString(null as Date)",
+			cql:        "ToString(null as Date)",
+			wantResult: newOrFatal(t, nil),
+		},
+		{
+			name:       "ToString(null)",
+			cql:        "ToString(null)",
+			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)
+			}
+		})
+	}
+}
diff --git a/tests/enginetests/setup.go b/tests/enginetests/setup.go
index 9abf26e..bda55e5 100644
--- a/tests/enginetests/setup.go
+++ b/tests/enginetests/setup.go
@@ -146,6 +146,9 @@ func addFHIRHelpersLib(t testing.TB, lib string) []string {
 	return []string{lib, string(fhirHelpers)}
 }
 
+// defaultInterpreterConfig returns an interpreter.Config with the default values used in the
+// engine tests.
+// The evaluation timestamp is fixed at Jan 1, 2024 +04:00.
 func defaultInterpreterConfig(t testing.TB, p *parser.Parser) interpreter.Config {
 	return interpreter.Config{
 		DataModels:          p.DataModel(),
diff --git a/tests/spectests/exclusions/exclusions.go b/tests/spectests/exclusions/exclusions.go
index 7ee12b8..cbf83b4 100644
--- a/tests/spectests/exclusions/exclusions.go
+++ b/tests/spectests/exclusions/exclusions.go
@@ -376,9 +376,12 @@ func XMLTestFileExclusionDefinitions() map[string]XMLTestFileExclusions {
 				"StartsWith",
 				"Substring",
 				"Upper",
-				"toString tests",
 			},
-			NamesExcludes: []string{},
+			NamesExcludes: []string{
+				// TODO: b/346880550 - These test appear to have incorrect assertions.
+				"DateTimeToString1",
+				"DateTimeToString2",
+			},
 		},
 		"CqlTypesTest.xml": XMLTestFileExclusions{
 			GroupExcludes: []string{},
@@ -400,7 +403,6 @@ func XMLTestFileExclusionDefinitions() map[string]XMLTestFileExclusions {
 				"ToBoolean",
 				"ToConcept",
 				"ToInteger",
-				"ToString",
 				"ToTime",
 			},
 			NamesExcludes: []string{