The concepts of a locally presentable category and an accessible category are extremely useful in formulating connections between universal algebra, model theory, logic, and computer science. The aim of this book is to provide an exposition of both the theory and the applications of these categories at a level accessible to graduate students. The concepts of lambda-presentable objects, locally lambda-presentable categories, and lambda-accessible categories are discussed in detail. The authors prove that Freyd's essentially algebraic categories are precisely the locally presentable categories. In the final chapter, they treat some advanced topics in model theory.