This chapter deals with a brief review of some basic principles of digital signals and processing in order to emphasize some fundamental features of digital systems. Logic gates are the essential components of combinational digital circuits and are discussed in terms of the main gate types and the properties of ideal gates, irrespective of how the gates are implemented. The essential component of sequential digital circuits is the latch, or its clocked version, the flip-flop. Memory circuits that store program instructions and data are essential elements of practically all digital systems. Digital signals are processed according to the rules of Boolean algebra, which is the algebra of two-valued logic. Various types of memory are almost universally used in digital systems to store and retrieve digital information. A read-only memory can be considered as a combinational digital circuit whose input is the address of a given memory location and whose output is the information that is stored in that location.