This book presents a 'bottom-up' approach to understanding decision making, starting from the fundamental question: what are the basic properties that a neural network of decision making needs to possess? It provides a general framework for the neurobiology of decision-making in vertebrates, and explains how it evolved from the lamprey to the apes. It also looks at the consequences of such a framework: how it impacts our capacity for reasoning, and considers some aspects of the pathophysiology of higher brain functions. Written in a lively and accessible style, the book presents an exciting perspective on understanding decision making.