Publisher Summary This chapter shows the embedded computer reading in signals, outputting control signals, interacting with a human user, and possibly interacting with an external system via a network. The important fact is that all of these activities fall under the broad arena of interfacing. This chapter shows a very large part of the hardware design of embedded systems. This discourse emphasizessome aspects of interfacing, including both the human and the physical interfaces. The chapter reveals that the human has to interface with any machine that he works with. This is almost certainly in some form of closed-loop contact. Diagrams of human and physical interfaces depict the domestic fridge, photocopier, and car dashboard. This chapter also shows that to interface with the physical world, the microcontroller must be able to interface with a range of transducers. It is necessary to create a mutual understanding between main sensors and actuators available and to keep abreast of current technology in the field. The chapter ends by disclosing that the interfacing with actuators requires a reasonable knowledge of power-switching techniques.