Session types are type-theoretic specifications of communication protocols in concurrent or distributed systems. By codifying the structure of communication, they make software more reliable and easier to construct. Over recent decades, the topic has become a large and active research area within the field of programming language theory and implementation. Written by leading researchers in the field, this is the first text to provide a comprehensive introduction to the key concepts of session types. The thorough theoretical treatment is complemented by examples and exercises, suitable for use in a lecture course or for self-study. It serves as an entry point to the topic for graduate students and researchers.