This book investigates an important but under-researched aspect of dictionary making: the use of a controlled vocabulary in definitions. The main concern of the author is the role of a definition vocabulary in how foreign learners understand and perceive dictionary definitions. The author takes the reader through a detailed historical account of controlled vocabularies and examines definitions in a range of English dictionaries with respect to their vocabulary loads. He performs a series of experiments with university students to reveal merits and shortcomings of restricted vocabularies. This monograph has been written with the aim to fill a gap in the literature on defining vocabulary. It is intended for lexicographers, dictionary editors, course designers, teachers, and students, as well as anyone who wishes to explain words in an intelligible way.