8051 Microcontrollers: MCS 51 Family and Its Variants is designed as a comprehensive textbook for undergraduate students of engineering. It provides a detailed exposure to product development using the 8-bit microcontroller MCS 51 and its variants. Starting from the fundamentals such as introduction to the architecture of the microcontroller chip, instruction set, and commonly used I/O and interfacing concepts; the text provides a large number of assembly language programming examples for proper understanding. A wide range of design applications are provided to introduce most of the concepts required for digital system design with microcontrollers. The most popular 8-bit microcontrollers used for designing intelligent digital instruments as well as controllers for a set of varied applications have been clearly explained and illustrated.