Publisher Summary This chapter focuses on different aspects of interfacing, including both the human and the physical interfaces. To design the interface one need to know about the sensors and actuators that can be used. These lie between the wider system and the electronic domain of the microcontroller circuit. There are input devices, sensors for measurement or data entry devices for human interaction. There are also output devices, displays or alarms for the human being, and motors or other actuators for the physical system. The chapter begins by discussing the needs of human interfacing and some simple means for meeting these needs. Whether or not the embedded microcontroller has a human interface, it certainly interfaces with the physical world. To do this it must be able to detect the state of physical variables and it must be able to control those variables. This interaction with the physical world is done by means of transducers. Input transducers, also called sensors, detect and convert physical variables into electrical variables. Output transducers convert electrical variables to physical.