This document provides an overview of transaction cost economics (TCE) and its application to the boundaries of the firm. It discusses how TCE, pioneered by Oliver Williamson, explains why firms vertically integrate to bring production stages in-house in order to reduce transaction costs, especially when asset specificity is present. The document also briefly describes alternative formal approaches to firm boundaries based on incomplete contracts, like the property rights theory of Grossman, Hart, and Moore, and relational contract theories.