This book, written by two pioneers in the field, provides a clear and concise description of memristors and other memory elements. It stresses the difference between their mathematical definition and physical reality. The reader will then be able to distinguish between what is experimentally realizable and various fictitious claims that plague the scientific literature. The discussion is kept simple enough that the book should be easily accessible not only to graduate students and researchers in Physics and Engineering, but also to undergraduate students interested in this topic.