This book gathers texts which give the best presentation of the principles and key concepts of the Theory of Didactical Situations that Guy Brousseau developed in the period from 1970 to 1990. These texts have been edited and organised so that they provide a comprehensive presentation of the Theory. Concepts such as didactical contract, didactical variable and epistemological obstacle are presented in detail. At the heart of the book, one will find two works which demonstrate in detail the articulation between theoretical work and experimental research which is the source of the richness of the Theory. Although referenced by many research texts, up to now the original texts of Guy Brousseau were unknown, or known only with difficulty, in the non- French-speaking world. This book is a response to these difficulties. In order to facilitate the reading of certain points, or elucidate them for the reader, footnotes have been added to the original text, as well as preludes and interludes to place in context the chosen texts and clarify the construction of the book.