This document presents an overview of Activity Theory in 20 slides. Activity Theory describes how human mental processes emerge from social interactions and are mediated by tools and language. It views human activities as hierarchical, consisting of goals, tasks, and operations. Activities take place in a social context that involves subjects, objects, community roles and rules. Activity Theory can be used as an analytical framework to understand user activities and ensure technology design is grounded in user needs, goals, and social contexts.