diff --git a/modelwriter.go b/modelwriter.go
index fd327e099..c6677ba2c 100644
--- a/modelwriter.go
+++ b/modelwriter.go
@@ -117,6 +117,7 @@ func (w *modelWriter) buildModelTransaction(out *model.Transaction, tx *Transact
 	out.Name = truncateString(td.Name)
 	out.Type = truncateString(td.Type)
 	out.Result = truncateString(td.Result)
+	out.Outcome = normalizeOutcome(td.Outcome)
 	out.Timestamp = model.Time(td.timestamp.UTC())
 	out.Duration = td.Duration.Seconds() * 1000
 	out.SpanCount.Started = td.spansCreated
@@ -151,6 +152,7 @@ func (w *modelWriter) buildModelSpan(out *model.Span, span *Span, sd *SpanData)
 	out.Action = truncateString(sd.Action)
 	out.Timestamp = model.Time(sd.timestamp.UTC())
 	out.Duration = sd.Duration.Seconds() * 1000
+	out.Outcome = normalizeOutcome(sd.Outcome)
 	out.Context = sd.Context.build()
 
 	// Copy the span type to context.destination.service.type.
@@ -271,3 +273,12 @@ func (w *modelWriter) setStacktraceContext(stack []model.StacktraceFrame) {
 		w.stats.Errors.SetContext++
 	}
 }
+
+func normalizeOutcome(outcome string) string {
+	switch outcome {
+	case "success", "failure", "unknown":
+		return outcome
+	default:
+		return "unknown"
+	}
+}
diff --git a/span.go b/span.go
index 85b244c1d..c34429c7f 100644
--- a/span.go
+++ b/span.go
@@ -394,6 +394,15 @@ type SpanData struct {
 	// duration based on the elapsed time since the span's start time.
 	Duration time.Duration
 
+	// Outcome holds the span outcome: success, failure, or unknown (the default).
+	// If Outcome is set to something else, it will be replaced with "unknown".
+	//
+	// Outcome is used for error rate calculations. A value of "success" indicates
+	// that a operation succeeded, while "failure" indicates that the operation
+	// failed. If Outcome is set to "unknown" (or some other value), then the
+	// span will not be included in error rate calculations.
+	Outcome string
+
 	// Context describes the context in which span occurs.
 	Context SpanContext
 
diff --git a/transaction.go b/transaction.go
index e5617fbe5..4db04e99b 100644
--- a/transaction.go
+++ b/transaction.go
@@ -322,6 +322,17 @@ type TransactionData struct {
 	// Result holds the transaction result.
 	Result string
 
+	// Outcome holds the transaction outcome: success, failure, or
+	// unknown (the default). If Outcome is set to something else,
+	// it will be replaced with "unknown".
+	//
+	// Outcome is used for error rate calculations. A value of "success"
+	// indicates that a transaction succeeded, while "failure" indicates
+	// that the transaction failed. If Outcome is set to "unknown" (or
+	// some other value), then the transaction will not be included in
+	// error rate calculations.
+	Outcome string
+
 	recording               bool
 	maxSpans                int
 	spanFramesMinDuration   time.Duration
diff --git a/validation_test.go b/validation_test.go
index 8df960f6d..d964a75b1 100644
--- a/validation_test.go
+++ b/validation_test.go
@@ -78,6 +78,14 @@ func TestValidateTransactionResult(t *testing.T) {
 	})
 }
 
+func TestValidateTransactionOutcome(t *testing.T) {
+	validatePayloads(t, func(tracer *apm.Tracer) {
+		tx := tracer.StartTransaction("name", "type")
+		tx.Outcome = "excellent" // changed to "unknown"
+		tx.End()
+	})
+}
+
 func TestValidateSpanName(t *testing.T) {
 	validateTransaction(t, func(tx *apm.Transaction) {
 		tx.StartSpan(strings.Repeat("x", 1025), "type", nil).End()
@@ -90,6 +98,14 @@ func TestValidateSpanType(t *testing.T) {
 	})
 }
 
+func TestValidateSpanOutcome(t *testing.T) {
+	validateTransaction(t, func(tx *apm.Transaction) {
+		span := tx.StartSpan("name", "type", nil)
+		span.Outcome = "excellent" // changed to "unknown"
+		span.End()
+	})
+}
+
 func TestValidateDatabaseSpanContext(t *testing.T) {
 	validateSpan(t, func(s *apm.Span) {
 		s.Context.SetDatabase(apm.DatabaseSpanContext{