diff --git a/core/block.go b/core/block.go
index 2d8a655..dad7ce9 100644
--- a/core/block.go
+++ b/core/block.go
@@ -6,31 +6,6 @@ import (
 	"time"
 )
 
-type Input struct {
-}
-type Output struct {
-}
-
-// Tx holds the data structure of a transaction
-type Tx struct {
-	From       Address
-	To         Address
-	InputCount uint64
-	Inputs     []Input
-	Outputs    []Output
-}
-
-func NewTx(from, to Address, in []Input, out []Output) *Tx {
-	tx := &Tx{
-		From:       from,
-		To:         to,
-		InputCount: uint64(len(in)),
-		Inputs:     in,
-		Outputs:    out,
-	}
-	return tx
-}
-
 // Block holds the data structure for the block
 type Block struct {
 	Height    uint64
diff --git a/core/block_test.go b/core/block_test.go
index 59c2274..7515e2e 100644
--- a/core/block_test.go
+++ b/core/block_test.go
@@ -59,13 +59,3 @@ func TestNewBlock(t *testing.T) {
 	// CheckPoW
 	assert.True(t, CheckPoW(h, difficulty))
 }
-
-func TestTx(t *testing.T) {
-	addr0 := Address(HashBytes([]byte("addr0")))
-	addr1 := Address(HashBytes([]byte("addr1")))
-
-	tx := NewTx(addr0, addr1, []Input{}, []Output{})
-
-	assert.Equal(t, tx.From, addr0)
-	assert.Equal(t, tx.To, addr1)
-}
diff --git a/core/tx.go b/core/tx.go
new file mode 100644
index 0000000..cc1d98f
--- /dev/null
+++ b/core/tx.go
@@ -0,0 +1,50 @@
+package core
+
+type Input struct {
+	TxId  Hash
+	Vout  int // index of the output from the TxId
+	Value uint64
+}
+type Output struct {
+	Value uint64
+}
+
+// Tx holds the data structure of a transaction
+type Tx struct {
+	From       Address
+	To         Address
+	InputCount uint64
+	Inputs     []Input
+	Outputs    []Output
+	Signature  []byte
+}
+
+func NewTx(from, to Address, in []Input, out []Output) *Tx {
+	tx := &Tx{
+		From:       from,
+		To:         to,
+		InputCount: uint64(len(in)),
+		Inputs:     in,
+		Outputs:    out,
+	}
+	return tx
+}
+
+// CheckTx checks if the transaction is consistent
+func CheckTx(tx *Tx) bool {
+	// TODO check that inputs and outputs are not empty
+
+	// check that inputs == outputs
+	totalIn := 0
+	for _, in := range tx.Inputs {
+		totalIn = totalIn + int(in.Value)
+	}
+	totalOut := 0
+	for _, out := range tx.Outputs {
+		totalOut = totalOut + int(out.Value)
+	}
+	if totalIn < totalOut {
+		return false
+	}
+	return true
+}
diff --git a/core/tx_test.go b/core/tx_test.go
new file mode 100644
index 0000000..5f250f8
--- /dev/null
+++ b/core/tx_test.go
@@ -0,0 +1,19 @@
+package core
+
+import (
+	"testing"
+
+	"github.com/stretchr/testify/assert"
+)
+
+func TestTx(t *testing.T) {
+	addr0 := Address(HashBytes([]byte("addr0")))
+	addr1 := Address(HashBytes([]byte("addr1")))
+
+	tx := NewTx(addr0, addr1, []Input{}, []Output{})
+
+	assert.Equal(t, tx.From, addr0)
+	assert.Equal(t, tx.To, addr1)
+
+	assert.True(t, CheckTx(tx))
+}