Taxonomy and terminology might seem like dull topics. But the diverse ways that eighteenth-century philosophers identified and classified the emotions (typically called “passions,” “affections,” or “sentiments”) crucially shaped the approaches they took. This chapter traces the sources available to eighteenth-century British philosophers for naming and ordering the passions, lays out the main vocabulary and concepts used for description and analysis, including the notions of “reflection” and “sympathy,” and outlines the principles that organized explanation, such as the division of the passions into the pleasurable or painful, and the selfish or social, as well as the role of “master passions.”