Book excerpt: Institutions are rules that are supported by various enforcement mechanisms. Cognition refers to the process of how men perceive and process information, whereas rationality refers to how these processes are modelled. Within institutional economics there is a growing scepticism towards extending the conventional economic frame of analysis to institutions. In particular, the notion of perfect rationality is increasingly questioned. At the same time human cognition has become a major field of research in psychology. This book explores what institutional economics can learn from cognitive psychology regarding the proper modelling of rationality in order to explain institutional change.