The lecture considers the process of the Brexit decision and negotiations from the time the decision was made to hold a referendum to when Parliament made one of the most far-reaching decisions in generations. The lecturer considers some general principles of rationality and reviews how some basic concepts in economics can illuminate the nature and procedures of the decision-making process by voters, politicians and government and EU negotiators. The focus on rationality concerns the process of decision-making rather than the actual decisions. Although a decision-making process may have elements of irrationality this does not necessarily mean that the final decision is irrational.