Equilibrium models used in beginning economics classes are based on the equilibrium concept developed by Alfred Marshall, but that concept of an equilibrium does not correspond to the equilibrium concept recognized in modern formal mathematical models taught to graduate students. In both cases, the assumptions needed to produce explanations of economic events are open to question. The assumptions needed to prove the existence of an equilibrium in formal mathematical models are often questioned not only by older model builders but also by today’s formal model builders. This book critically examines both model building cultures by examining the major problematic assumptions employed building equilibrium models with particular attention to the assumptions used to characterize learning, knowledge, and expectations. These assumptions are recognized as essential in any equilibrium model that claims to address the dynamics of decision making. These assumptions are also the object of the critiques provided by those developing evolutionary models and by those promoting the development of complexity economics. Attention is also given to the inadequacies of what is taught to beginning students when it comes to the question of whether equilibrium models can provide a realistic explanation of economic events and objects such as prices, market demands, and market supplies.