APOS Theory is a specialized book that focuses on the APOS Theory, a theoretical approach to the construction of mathematical concepts. Despite the extensive application of this theory in numerous scientific publications, textbook designs, and teaching practices, there has yet to be a comprehensive reference work that summarizes all relevant information about its components and provides guidance on its application. This book aims to present the essential elements of the APOS Theory and is directed at researchers who are engaged with this approach or wish to learn more about it, as well as individuals interested in the construction of mathematical concepts according to this theory. It is particularly useful for researchers in mathematics education, graduate students in mathematics education, and mathematics instructors.