This book concerns particles that are used as responses in conversations. It provides much needed methodological tools for analyzing the use of response particles in languages, while its particular focus is Finnish. The book focuses on two Finnish particles, nii(n) and joo, which in some of their central usages have “yeah” and “yes” as their closest English counterparts. The two particles are discussed in a number of sequential and activity contexts, including their use as answers to yes-no questions and directives, as responses to a stance-taking by the prior speaker, and in the midst of an extended telling by the co-participant. It will be shown how there is a fine-grained division of labor between the particles, having to do with the epistemic and affective character of the talk and the continuation vs. closure-relevance of the activity. The book connects the interactional usages of the particles with what is known about their historical origins, and in this fashion it is also of interest to linguists doing research on processes of grammaticalization and lexicalization.