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 it be the configuration information or some sampled data to send back to the controller. The controller must write information to these registers in order to instruct the target device to perform a task.

While it is common to have registers in I²C targets, note that not all target devices will have registers. Some devices are simple and contain only 1 register, which may be written to directly by sending the register data immediately after the target address, instead of addressing a register. An example of a single-register device would be an 8-bit I²C switch, which is controlled via I²C commands. Since it 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.