This chapter provides an introduction to the phenomenon of prosodic focus, as well as to the theory of Alternative Semantics. Alternative Semantics provides an insightful account of what prosodic focus means, and gives us a notation that can help with better characterizing focus-related phenomena and the terminology used to describe them. We can also translate theoretical ideas about focus and givenness into this notation to facilitate a comparison between frameworks. The discussion will partly be structured by an evaluation of the theories of Givenness, the theory of Relative Givenness, and Unalternative Semantics, but we will cover a range of other ideas and proposals in the process. The chapter concludes with a discussion of phonological issues, and of association with focus.