diff --git a/go.mod b/go.mod
index 02740b6..a66a993 100644
--- a/go.mod
+++ b/go.mod
@@ -2,4 +2,7 @@ module tinygo.org/x/drivers
 
 go 1.14
 
-require github.com/eclipse/paho.mqtt.golang v1.2.0
+require (
+	github.com/eclipse/paho.mqtt.golang v1.2.0
+	github.com/frankban/quicktest v1.10.2
+)
diff --git a/go.sum b/go.sum
index 9dfbb0b..2c57487 100644
--- a/go.sum
+++ b/go.sum
@@ -1,2 +1,13 @@
 github.com/eclipse/paho.mqtt.golang v1.2.0 h1:1F8mhG9+aO5/xpdtFkW4SxOJB67ukuDC3t2y2qayIX0=
 github.com/eclipse/paho.mqtt.golang v1.2.0/go.mod h1:H9keYFcgq3Qr5OUJm/JZI/i6U7joQ8SYLhZwfeOo6Ts=
+github.com/frankban/quicktest v1.10.2 h1:19ARM85nVi4xH7xPXuc5eM/udya5ieh7b/Sv+d844Tk=
+github.com/frankban/quicktest v1.10.2/go.mod h1:K+q6oSqb0W0Ininfk863uOk1lMy69l/P6txr3mVT54s=
+github.com/google/go-cmp v0.5.2 h1:X2ev0eStA3AbceY54o37/0PQ/UWqKEiiO2dKL5OPaFM=
+github.com/google/go-cmp v0.5.2/go.mod h1:v8dTdLbMG2kIc/vJvl+f65V22dbkXbowE6jgT/gNBxE=
+github.com/kr/pretty v0.2.1 h1:Fmg33tUaq4/8ym9TJN1x7sLJnHVwhP33CNkpYV/7rwI=
+github.com/kr/pretty v0.2.1/go.mod h1:ipq/a2n7PKx3OHsz4KJII5eveXtPO4qwEXGdVfWzfnI=
+github.com/kr/pty v1.1.1/go.mod h1:pFQYn66WHrOpPYNljwOMqo10TkYh1fy3cYio2l3bCsQ=
+github.com/kr/text v0.1.0 h1:45sCR5RtlFHMR4UwH9sdQ5TC8v0qDQCHnXt+kaKSTVE=
+github.com/kr/text v0.1.0/go.mod h1:4Jbv+DJW3UT/LiOwJeYQe1efqtUx/iVham/4vfdArNI=
+golang.org/x/xerrors v0.0.0-20191204190536-9bdfabe68543 h1:E7g+9GITq07hpfrRu66IVDexMakfv52eLZ2CXBWiKr4=
+golang.org/x/xerrors v0.0.0-20191204190536-9bdfabe68543/go.mod h1:I/5z698sn9Ka8TeJc9MKroUUfqBBauWjQqLJ2OPfmY0=
diff --git a/i2c.go b/i2c.go
new file mode 100644
index 0000000..27bc27d
--- /dev/null
+++ b/i2c.go
@@ -0,0 +1,9 @@
+package drivers
+
+// I2C represents an I2C bus. It is notably implemented by the
+// machine.I2C type.
+type I2C interface {
+	ReadRegister(addr uint8, r uint8, buf []byte) error
+	WriteRegister(addr uint8, r uint8, buf []byte) error
+	Tx(addr uint16, w, r []byte) error
+}
diff --git a/tester/device.go b/tester/device.go
new file mode 100644
index 0000000..43b669b
--- /dev/null
+++ b/tester/device.go
@@ -0,0 +1,58 @@
+package tester
+
+import (
+	qt "github.com/frankban/quicktest"
+)
+
+const maxRegisters = 200
+
+// I2CDevice represents a mock I2C device on a mock I2C bus.
+type I2CDevice struct {
+	C    *qt.C
+	Addr uint8
+	// Registers holds the device registers. It can be inspected
+	// or changed as desired for testing.
+	Registers [maxRegisters]uint8
+	// If Err is non-nil, it will be returned as the error from the
+	// I2C methods.
+	Err error
+}
+
+// NewI2CDevice returns a new mock I2C device.
+func NewI2CDevice(c *qt.C, addr uint8) *I2CDevice {
+	return &I2CDevice{
+		C:    c,
+		Addr: addr,
+	}
+}
+
+// ReadRegister implements I2C.ReadRegister.
+func (d *I2CDevice) ReadRegister(r uint8, buf []byte) error {
+	if d.Err != nil {
+		return d.Err
+	}
+	d.AssertRegisterRange(r, buf)
+	copy(buf, d.Registers[r:])
+	return nil
+}
+
+// WriteRegister implements I2C.WriteRegister.
+func (d *I2CDevice) WriteRegister(r uint8, buf []byte) error {
+	if d.Err != nil {
+		return d.Err
+	}
+	d.AssertRegisterRange(r, buf)
+	copy(d.Registers[r:], buf)
+	return nil
+}
+
+// AssertRegisterRange asserts that reading or writing the given
+// register and subsequent registers is in range of the available registers.
+func (d *I2CDevice) AssertRegisterRange(r uint8, buf []byte) {
+	if int(r) >= len(d.Registers) {
+		d.C.Fatalf("register read/write [%#x, %#x] start out of range", r, int(r)+len(buf))
+	}
+	if int(r)+len(buf) > len(d.Registers) {
+		d.C.Fatalf("register read/write [%#x, %#x] end out of range", r, int(r)+len(buf))
+	}
+}
diff --git a/tester/i2c.go b/tester/i2c.go
new file mode 100644
index 0000000..79887c3
--- /dev/null
+++ b/tester/i2c.go
@@ -0,0 +1,52 @@
+package tester
+
+import (
+	qt "github.com/frankban/quicktest"
+)
+
+// I2CBus implements the I2C interface in memory for testing.
+type I2CBus struct {
+	C       *qt.C
+	Devices []*I2CDevice
+}
+
+// NewI2CBus returns an I2CBus mock I2C instance that uses c to flag errors
+// if they happen. After creating a I2C instance, add devices
+// to it with addDevice before using NewI2CBus interface.
+func NewI2CBus(c *qt.C) *I2CBus {
+	return &I2CBus{
+		C: c,
+	}
+}
+
+// AddDevice adds a new mock device to the mock I2C bus.
+func (bus *I2CBus) AddDevice(d *I2CDevice) {
+	bus.Devices = append(bus.Devices, d)
+}
+
+// ReadRegister implements I2C.ReadRegister.
+func (bus *I2CBus) ReadRegister(addr uint8, r uint8, buf []byte) error {
+	return bus.FindDevice(addr).ReadRegister(r, buf)
+}
+
+// WriteRegister implements I2C.WriteRegister.
+func (bus *I2CBus) WriteRegister(addr uint8, r uint8, buf []byte) error {
+	return bus.FindDevice(addr).WriteRegister(r, buf)
+}
+
+// Tx implements I2C.Tx.
+func (bus *I2CBus) Tx(addr uint16, input, output []byte) error {
+	// TODO: implement this
+	return nil
+}
+
+// FindDevice returns the device with the given address.
+func (bus *I2CBus) FindDevice(addr uint8) *I2CDevice {
+	for _, dev := range bus.Devices {
+		if dev.Addr == addr {
+			return dev
+		}
+	}
+	bus.C.Fatalf("invalid device addr %#x passed to i2c bus", addr)
+	panic("unreachable")
+}
diff --git a/tester/tester.go b/tester/tester.go
new file mode 100644
index 0000000..5caf27e
--- /dev/null
+++ b/tester/tester.go
@@ -0,0 +1,5 @@
+// Package tester contains mock structs to make it easier to test I2C devices.
+//
+// TODO: info on how to use this.
+//
+package tester // import "tinygo.org/x/drivers/tester"