This chapter examines the design challenges and requirements for ion sources and extraction systems that are used for ion implantation. The evolution of the indirectly heated cathode ion source, which is the most common source in use today, will be reviewed. The physics, function, operation, and general use of this and other sources will be discussed in some detail, with a goal of not only allowing a good understanding of the design but also to present operational and maintenance tips designed to maximize source life and optimize beam quality. Sources for selected implantation applications will also be described, along with the features that make them well suited for those applications.