diff --git a/crypto/multisig/compact_bit_array.go b/crypto/multisig/compact_bit_array.go
index ff6ecd646..94e9791cd 100644
--- a/crypto/multisig/compact_bit_array.go
+++ b/crypto/multisig/compact_bit_array.go
@@ -71,6 +71,19 @@ func (bA *CompactBitArray) SetIndex(i int, v bool) bool {
 	return true
 }
 
+// NumTrueBitsBefore returns the number of bits set to true before the
+// given index. e.g. if bA = _XX__XX, NumOfTrueBitsBefore(4) = 2, since
+// there are two bits set to true before index 4.
+func (bA *CompactBitArray) NumTrueBitsBefore(index int) int {
+	numTrueValues := 0
+	for i := 0; i < index; i++ {
+		if bA.GetIndex(i) {
+			numTrueValues++
+		}
+	}
+	return numTrueValues
+}
+
 // Copy returns a copy of the provided bit array.
 func (bA *CompactBitArray) Copy() *CompactBitArray {
 	if bA == nil {
diff --git a/crypto/multisig/compact_bit_array_test.go b/crypto/multisig/compact_bit_array_test.go
index 91a82192f..8b342b7ad 100644
--- a/crypto/multisig/compact_bit_array_test.go
+++ b/crypto/multisig/compact_bit_array_test.go
@@ -150,6 +150,32 @@ func TestCompactMarshalUnmarshal(t *testing.T) {
 	}
 }
 
+func TestCompactBitArrayNumOfTrueBitsBefore(t *testing.T) {
+	testCases := []struct {
+		marshalledBA   string
+		bAIndex        []int
+		trueValueIndex []int
+	}{
+		{`"_____"`, []int{0, 1, 2, 3, 4}, []int{0, 0, 0, 0, 0}},
+		{`"x"`, []int{0}, []int{0}},
+		{`"_x"`, []int{1}, []int{0}},
+		{`"x___xxxx"`, []int{0, 4, 5, 6, 7}, []int{0, 1, 2, 3, 4}},
+		{`"__x_xx_x__x_x___"`, []int{2, 4, 5, 7, 10, 12}, []int{0, 1, 2, 3, 4, 5}},
+		{`"______________xx"`, []int{14, 15}, []int{0, 1}},
+	}
+	for tcIndex, tc := range testCases {
+		t.Run(tc.marshalledBA, func(t *testing.T) {
+			var bA *CompactBitArray
+			err := json.Unmarshal([]byte(tc.marshalledBA), &bA)
+			require.NoError(t, err)
+
+			for i := 0; i < len(tc.bAIndex); i++ {
+				require.Equal(t, tc.trueValueIndex[i], bA.NumTrueBitsBefore(tc.bAIndex[i]), "tc %d, i %d", tcIndex, i)
+			}
+		})
+	}
+}
+
 func TestCompactBitArrayGetSetIndex(t *testing.T) {
 	r := rand.New(rand.NewSource(100))
 	numTests := 10
diff --git a/crypto/multisig/multisignature.go b/crypto/multisig/multisignature.go
new file mode 100644
index 000000000..29e8a30b9
--- /dev/null
+++ b/crypto/multisig/multisignature.go
@@ -0,0 +1,69 @@
+package multisig
+
+import (
+	"errors"
+
+	"github.com/tendermint/tendermint/crypto"
+)
+
+// Multisignature is used to represent the signature object used in the multisigs.
+// Sigs is a list of signatures, sorted by corresponding index.
+type Multisignature struct {
+	BitArray *CompactBitArray
+	Sigs     [][]byte
+}
+
+// NewMultisig returns a new Multisignature of size n.
+func NewMultisig(n int) *Multisignature {
+	// Default the signature list to have a capacity of two, since we can
+	// expect that most multisigs will require multiple signers.
+	return &Multisignature{NewCompactBitArray(n), make([][]byte, 0, 2)}
+}
+
+// GetIndex returns the index of pk in keys. Returns -1 if not found
+func getIndex(pk crypto.PubKey, keys []crypto.PubKey) int {
+	for i := 0; i < len(keys); i++ {
+		if pk.Equals(keys[i]) {
+			return i
+		}
+	}
+	return -1
+}
+
+// AddSignature adds a signature to the multisig, at the corresponding index.
+// If the signature already exists, replace it.
+func (mSig *Multisignature) AddSignature(sig []byte, index int) {
+	newSigIndex := mSig.BitArray.NumTrueBitsBefore(index)
+	// Signature already exists, just replace the value there
+	if mSig.BitArray.GetIndex(index) {
+		mSig.Sigs[newSigIndex] = sig
+		return
+	}
+	mSig.BitArray.SetIndex(index, true)
+	// Optimization if the index is the greatest index
+	if newSigIndex == len(mSig.Sigs) {
+		mSig.Sigs = append(mSig.Sigs, sig)
+		return
+	}
+	// Expand slice by one with a dummy element, move all elements after i
+	// over by one, then place the new signature in that gap.
+	mSig.Sigs = append(mSig.Sigs, make([]byte, 0))
+	copy(mSig.Sigs[newSigIndex+1:], mSig.Sigs[newSigIndex:])
+	mSig.Sigs[newSigIndex] = sig
+}
+
+// AddSignatureFromPubkey adds a signature to the multisig,
+// at the index in keys corresponding to the provided pubkey.
+func (mSig *Multisignature) AddSignatureFromPubkey(sig []byte, pubkey crypto.PubKey, keys []crypto.PubKey) error {
+	index := getIndex(pubkey, keys)
+	if index == -1 {
+		return errors.New("provided key didn't exist in pubkeys")
+	}
+	mSig.AddSignature(sig, index)
+	return nil
+}
+
+// Marshal the multisignature with amino
+func (mSig *Multisignature) Marshal() []byte {
+	return cdc.MustMarshalBinaryBare(mSig)
+}
diff --git a/crypto/multisig/threshold_multisig.go b/crypto/multisig/threshold_multisig.go
new file mode 100644
index 000000000..58a3ea0e3
--- /dev/null
+++ b/crypto/multisig/threshold_multisig.go
@@ -0,0 +1,92 @@
+package multisig
+
+import (
+	"github.com/tendermint/tendermint/crypto"
+	"github.com/tendermint/tendermint/crypto/tmhash"
+)
+
+// ThresholdMultiSignaturePubKey implements a K of N threshold multisig.
+type ThresholdMultiSignaturePubKey struct {
+	K       uint            `json:"threshold"`
+	Pubkeys []crypto.PubKey `json:"pubkeys"`
+}
+
+var _ crypto.PubKey = &ThresholdMultiSignaturePubKey{}
+
+// NewThresholdMultiSignaturePubKey returns a new ThresholdMultiSignaturePubKey.
+// Panics if len(pubkeys) < k or 0 >= k.
+func NewThresholdMultiSignaturePubKey(k int, pubkeys []crypto.PubKey) crypto.PubKey {
+	if k <= 0 {
+		panic("threshold k of n multisignature: k <= 0")
+	}
+	if len(pubkeys) < k {
+		panic("threshold k of n multisignature: len(pubkeys) < k")
+	}
+	return &ThresholdMultiSignaturePubKey{uint(k), pubkeys}
+}
+
+// VerifyBytes expects sig to be an amino encoded version of a MultiSignature.
+// Returns true iff the multisignature contains k or more signatures
+// for the correct corresponding keys,
+// and all signatures are valid. (Not just k of the signatures)
+// The multisig uses a bitarray, so multiple signatures for the same key is not
+// a concern.
+func (pk *ThresholdMultiSignaturePubKey) VerifyBytes(msg []byte, marshalledSig []byte) bool {
+	var sig *Multisignature
+	err := cdc.UnmarshalBinaryBare(marshalledSig, &sig)
+	if err != nil {
+		return false
+	}
+	size := sig.BitArray.Size()
+	// ensure bit array is the correct size
+	if len(pk.Pubkeys) != size {
+		return false
+	}
+	// ensure size of signature list
+	if len(sig.Sigs) < int(pk.K) || len(sig.Sigs) > size {
+		return false
+	}
+	// ensure at least k signatures are set
+	if sig.BitArray.NumTrueBitsBefore(size) < int(pk.K) {
+		return false
+	}
+	// index in the list of signatures which we are concerned with.
+	sigIndex := 0
+	for i := 0; i < size; i++ {
+		if sig.BitArray.GetIndex(i) {
+			if !pk.Pubkeys[i].VerifyBytes(msg, sig.Sigs[sigIndex]) {
+				return false
+			}
+			sigIndex++
+		}
+	}
+	return true
+}
+
+// Bytes returns the amino encoded version of the ThresholdMultiSignaturePubKey
+func (pk *ThresholdMultiSignaturePubKey) Bytes() []byte {
+	return cdc.MustMarshalBinaryBare(pk)
+}
+
+// Address returns tmhash(ThresholdMultiSignaturePubKey.Bytes())
+func (pk *ThresholdMultiSignaturePubKey) Address() crypto.Address {
+	return crypto.Address(tmhash.Sum(pk.Bytes()))
+}
+
+// Equals returns true iff pk and other both have the same number of keys, and
+// all constituent keys are the same, and in the same order.
+func (pk *ThresholdMultiSignaturePubKey) Equals(other crypto.PubKey) bool {
+	otherKey, sameType := other.(*ThresholdMultiSignaturePubKey)
+	if !sameType {
+		return false
+	}
+	if pk.K != otherKey.K || len(pk.Pubkeys) != len(otherKey.Pubkeys) {
+		return false
+	}
+	for i := 0; i < len(pk.Pubkeys); i++ {
+		if !pk.Pubkeys[i].Equals(otherKey.Pubkeys[i]) {
+			return false
+		}
+	}
+	return true
+}
diff --git a/crypto/multisig/threshold_multisig_test.go b/crypto/multisig/threshold_multisig_test.go
new file mode 100644
index 000000000..2d2fdeec8
--- /dev/null
+++ b/crypto/multisig/threshold_multisig_test.go
@@ -0,0 +1,112 @@
+package multisig
+
+import (
+	"math/rand"
+	"testing"
+
+	"github.com/stretchr/testify/require"
+
+	"github.com/tendermint/tendermint/crypto"
+	"github.com/tendermint/tendermint/crypto/ed25519"
+	"github.com/tendermint/tendermint/crypto/secp256k1"
+)
+
+// This tests multisig functionality, but it expects the first k signatures to be valid
+// TODO: Adapt it to give more flexibility about first k signatures being valid
+func TestThresholdMultisigValidCases(t *testing.T) {
+	pkSet1, sigSet1 := generatePubKeysAndSignatures(5, []byte{1, 2, 3, 4})
+	cases := []struct {
+		msg            []byte
+		k              int
+		pubkeys        []crypto.PubKey
+		signingIndices []int
+		// signatures should be the same size as signingIndices.
+		signatures           [][]byte
+		passAfterKSignatures []bool
+	}{
+		{
+			msg:                  []byte{1, 2, 3, 4},
+			k:                    2,
+			pubkeys:              pkSet1,
+			signingIndices:       []int{0, 3, 1},
+			signatures:           sigSet1,
+			passAfterKSignatures: []bool{false},
+		},
+	}
+	for tcIndex, tc := range cases {
+		multisigKey := NewThresholdMultiSignaturePubKey(tc.k, tc.pubkeys)
+		multisignature := NewMultisig(len(tc.pubkeys))
+		for i := 0; i < tc.k-1; i++ {
+			signingIndex := tc.signingIndices[i]
+			multisignature.AddSignatureFromPubkey(tc.signatures[signingIndex], tc.pubkeys[signingIndex], tc.pubkeys)
+			require.False(t, multisigKey.VerifyBytes(tc.msg, multisignature.Marshal()),
+				"multisig passed when i < k, tc %d, i %d", tcIndex, i)
+			multisignature.AddSignatureFromPubkey(tc.signatures[signingIndex], tc.pubkeys[signingIndex], tc.pubkeys)
+			require.Equal(t, i+1, len(multisignature.Sigs),
+				"adding a signature for the same pubkey twice increased signature count by 2, tc %d", tcIndex)
+		}
+		require.False(t, multisigKey.VerifyBytes(tc.msg, multisignature.Marshal()),
+			"multisig passed with k - 1 sigs, tc %d", tcIndex)
+		multisignature.AddSignatureFromPubkey(tc.signatures[tc.signingIndices[tc.k]], tc.pubkeys[tc.signingIndices[tc.k]], tc.pubkeys)
+		require.True(t, multisigKey.VerifyBytes(tc.msg, multisignature.Marshal()),
+			"multisig failed after k good signatures, tc %d", tcIndex)
+		for i := tc.k + 1; i < len(tc.signingIndices); i++ {
+			signingIndex := tc.signingIndices[i]
+			multisignature.AddSignatureFromPubkey(tc.signatures[signingIndex], tc.pubkeys[signingIndex], tc.pubkeys)
+			require.Equal(t, tc.passAfterKSignatures[i-tc.k-1],
+				multisigKey.VerifyBytes(tc.msg, multisignature.Marshal()),
+				"multisig didn't verify as expected after k sigs, tc %d, i %d", tcIndex, i)
+
+			multisignature.AddSignatureFromPubkey(tc.signatures[signingIndex], tc.pubkeys[signingIndex], tc.pubkeys)
+			require.Equal(t, i+1, len(multisignature.Sigs),
+				"adding a signature for the same pubkey twice increased signature count by 2, tc %d", tcIndex)
+		}
+	}
+}
+
+// TODO: Fully replace this test with table driven tests
+func TestThresholdMultisigDuplicateSignatures(t *testing.T) {
+	msg := []byte{1, 2, 3, 4, 5}
+	pubkeys, sigs := generatePubKeysAndSignatures(5, msg)
+	multisigKey := NewThresholdMultiSignaturePubKey(2, pubkeys)
+	multisignature := NewMultisig(5)
+	require.False(t, multisigKey.VerifyBytes(msg, multisignature.Marshal()))
+	multisignature.AddSignatureFromPubkey(sigs[0], pubkeys[0], pubkeys)
+	// Add second signature manually
+	multisignature.Sigs = append(multisignature.Sigs, sigs[0])
+	require.False(t, multisigKey.VerifyBytes(msg, multisignature.Marshal()))
+}
+
+// TODO: Fully replace this test with table driven tests
+func TestMultiSigPubkeyEquality(t *testing.T) {
+	msg := []byte{1, 2, 3, 4}
+	pubkeys, _ := generatePubKeysAndSignatures(5, msg)
+	multisigKey := NewThresholdMultiSignaturePubKey(2, pubkeys)
+	var unmarshalledMultisig *ThresholdMultiSignaturePubKey
+	cdc.MustUnmarshalBinaryBare(multisigKey.Bytes(), &unmarshalledMultisig)
+	require.True(t, multisigKey.Equals(unmarshalledMultisig))
+
+	// Ensure that reordering pubkeys is treated as a different pubkey
+	pubkeysCpy := make([]crypto.PubKey, 5)
+	copy(pubkeysCpy, pubkeys)
+	pubkeysCpy[4] = pubkeys[3]
+	pubkeysCpy[3] = pubkeys[4]
+	multisigKey2 := NewThresholdMultiSignaturePubKey(2, pubkeysCpy)
+	require.False(t, multisigKey.Equals(multisigKey2))
+}
+
+func generatePubKeysAndSignatures(n int, msg []byte) (pubkeys []crypto.PubKey, signatures [][]byte) {
+	pubkeys = make([]crypto.PubKey, n)
+	signatures = make([][]byte, n)
+	for i := 0; i < n; i++ {
+		var privkey crypto.PrivKey
+		if rand.Int63()%2 == 0 {
+			privkey = ed25519.GenPrivKey()
+		} else {
+			privkey = secp256k1.GenPrivKey()
+		}
+		pubkeys[i] = privkey.PubKey()
+		signatures[i], _ = privkey.Sign(msg)
+	}
+	return
+}
diff --git a/crypto/multisig/wire.go b/crypto/multisig/wire.go
new file mode 100644
index 000000000..a6cda34a9
--- /dev/null
+++ b/crypto/multisig/wire.go
@@ -0,0 +1,26 @@
+package multisig
+
+import (
+	amino "github.com/tendermint/go-amino"
+	"github.com/tendermint/tendermint/crypto"
+	"github.com/tendermint/tendermint/crypto/ed25519"
+	"github.com/tendermint/tendermint/crypto/secp256k1"
+)
+
+// TODO: Figure out API for others to either add their own pubkey types, or
+// to make verify / marshal accept a cdc.
+const (
+	ThresholdPubkeyAminoRoute = "tendermint/PubkeyThresholdMultisig"
+)
+
+var cdc = amino.NewCodec()
+
+func init() {
+	cdc.RegisterInterface((*crypto.PubKey)(nil), nil)
+	cdc.RegisterConcrete(ThresholdMultiSignaturePubKey{},
+		ThresholdPubkeyAminoRoute, nil)
+	cdc.RegisterConcrete(ed25519.PubKeyEd25519{},
+		ed25519.Ed25519PubKeyAminoRoute, nil)
+	cdc.RegisterConcrete(secp256k1.PubKeySecp256k1{},
+		secp256k1.Secp256k1PubKeyAminoRoute, nil)
+}