The common explanation for the expansion of bureaus is based on rationalist reasoning. Bureaus grow because it is in the interest of the central decision makers to have them grow. This article presents a different explanation, drawing on the organizational-processes literature. It argues that administrative growth occurs as a byproduct of managers dealing with more immediate concerns, not because it is pursued for its own value. Part of the explanation includes arguments for the highly reactive and highly interactive nature of managerial work. The ways in which the combined behaviors of individual managers translate into organizational expansion are illustrated using data comprised of 29,640 random observations of 52 managers at work.