The worldwide management consultancy industry is estimated to be worth over $300 billion annually. Its importance, in terms of the global economy, cannot be overestimated. But what is management consultancy? The thesis addresses this question by drawing on the work of Michel Foucault and applying his diagnostic method to the field of management consultancy knowledge. Managerial advisory services take place in a variety of operational areas, such as business strategy, human resources, manufacturing, marketing, and finance. But there is a type of knowledge that remains the same for all management consultants, irrespective of the specific operational area in which they work. This knowledge is found in the practitioner literature for management consultants and takes the form of prescriptive guidelines for dealing with managers in the client organization. Some practitioners have begun to suggest that these prescriptive guidelines can be used by managers as well as consultants. Instead of dealing with their subordinates in the traditional way, managers are now being told to act as 'their own consultants' in relation to their 'clients'. This means that management consultancy knowledge is changing the way we understand the function of modern management. The thesis brings to light the field of management consultancy knowledge by describing the formation of its objects, concepts, and diverging theories. It also examines the institutional effects of the practitioner literature in terms of the profession of management consultancy. By outlining the relations between knowledge and power, the thesis answers the question of what management consultancy is.