Over the past two decades, there has been an extraordinary expansion of the economic theory of the trade union. There is also a huge empirical literature describing and quantifying the impact of the trade union on a host of labour market outcomes. The purpose of this book is to impose some structure on this body of literature, in order to make it easily accessible to the student of the economics of the trade union or the economics of industrial relations. The book is not intended to be a comprehensive survey of the literature. Rather, it aims to provide a selective review of the crucial features of the analytical framework used by economists in modelling trade unions, a framework that is also potentially applicable to nonunionised labour markets where workers have some market power. Certain issues have been deliberately omitted in the interests of keeping the book to a manageable size for the readership for which it is intended. The book aims to be accessible in the main to third-year undergraduates and MSc students in economics, as well as to general readers with some basic training in economics and quantitative techniques. The main technical parts of the analytical framework have been confined to appendixes where appropriate, in order to avoid distracting the reader's attention from the main themes.