If participants in a conversation wish to avoid using redundant words, they often can. Although this phenomenon of ellipsis, which was illustrated the previous sentence (some words are missing after ‘can’) is ubiquitous in natural language, it cannot occur freely: it must obey certain grammatical and pragmatic rules. But what are these rules, why do they hold in the first place, and why do some of them differ across languages? From the perspective of linguistic theory, what exactly IS ellipsis, anyway? Focusing primarily on English, this course outlines some of the answers that have been offered to these questions.