The purpose of this book is to provide comprehensive information of the 8051 microcontroller and interfacing programming. Simplicity and stepwise flow with sufficient details and neat diagrams are the features of this book. It includes the details of 8051 uC & I/O devices interfacing in Assembly and C language.This book has nine chapters. The 1st describes the difference of uP & uC based system, Von Neumann & Harvard architecture & details of 8051 uC. The 2nd chapter deals with the special function registers, their functions and its operation. Assembly language programming, instruction set, addressing modes, development tools & ALP programs are discussed in the 3rd chapter. Chapter 4 gives the details of 8051 timer/counters and its related SFRs- TCON & TMOD. Fifth chapter discusses the basic of serial communication and the SFRs-SCON & SBUF used for communication. Input devices (switch, TWS, KB) and output devices (LED, 7-segment display, LCD, relay, DC and stepper motor, optocoupler) i