This chapter is dedicated to a discussion on computer arithmetic. It introduces the fundamental issues related to the arithmetic operations and circuits used to support computation in computers. Our coverage starts with an introduction to number systems. In particular, we introduce issues such as number representations and base conversion. This is followed by a discussion on integer arithmetic. In this regard, we introduce a number of algorithms together with hardware schemes that are used in performing integer addition, subtraction, multiplication, and division. We end up our discussion in this chapter with a discussion on floating-point arithmetic including floating-point representation, floating-point operations, and floating-point hardware schemes. The IEEE floating-point standard is the last topic discussed in the chapter.