Thisisthe?rstbookinworldliteraturedevotedtoapplicationsoftheisotopic e?ect in solids. It is accessible to physicists, chemists, electronic engineers, and material scientists alike. This book is intended both as tutorial and as reference. Readers seeking to learn the basics of application of the isotopic e?ect in solids should start by reading the ?rst few overview chapters, and then dig into descriptions of speci?c applications to see how they really work. It is hoped that it will be useful to undergraduate and graduate students of physics and optics as well as engineers, physicists, material scientists, and medical doctors who are interested in investigation or applications of the isotopic e?ect in solids. This book is a state-of-the-art introduction to very recent activity in solid-state physics which has developed in the main during the last half century and promises a new technology of isotopic engineering. Important applications are to be expected for information storage and dev- oping materials for computer memory, quantum computers, isotopic ?bers, isotopic optoelectronics, and quantum electronics as well as UV lasers. The references I cite are those with which I am most familiar and which have helped us understand the subject as presented here. There has been no attempttogivecredittoeachcontributor,butIhavetriedtocitetheoriginal papers, which brought new and important results (methods) to applications of the isotopic e?ect in solids, covered in this text.