Data must be sent to and received from the target devices, and this is accomplished by reading from or writing to registers in the target device.

Registers are locations in the memory of the target which contain information, whether configuration information or some sampled data to send back to the controller. The controller must write information to these registers to instruct the target device to perform a task.

While having registers in I²C targets is common, note that not all target devices have registers. Some devices are simple and contain only 1 register, which can be written to directly by sending the register data immediately after the target address, instead of addressing a register. The TCA9847 is an example of a single-register device, which is controlled through I²C commands. Because the device has 1 bit to enable or disable a channel, there is only 1 register needed, and the controller merely writes the register data after the target address, skipping the register number.