Summary: This glossary includes self-contained definitions of most complexity classes mentioned in the book. Needless to say, the glossary offers a very minimal discussion of these classes, and the reader is referred to the main text for further discussion. The items are organized by topics rather than by alphabetic order. Specifically, the glossary is partitioned into two parts, dealing separately with complexity classes that are defined in terms of algorithms and their resources (i.e., time and space complexity of Turing machines) and complexity classes defined in terms of non-uniform circuits (and referring to their size and depth). The algorithmic classes include time complexity classes (such as P, NP, coNP, BPP, RP, coRP, PH, ε, εχP, and NεχP) and the space complexity classes, ℒ, Nℒ, Rℒ, and PSP ACε. The non-uniform classes include the circuit classes P/poly as well as NCk and ACk.