This monograph summarizes the recent major achievements in Möbius invariant QK spaces. First introduced by Hasi Wulan and his collaborators, the theory of QK spaces has developed immensely in the last two decades, and the topics covered in this book will be helpful to graduate students and new researchers interested in the field. Featuring a wide range of subjects, including an overview of QK spaces, QK-Teichmüller spaces, K-Carleson measures and analysis of weight functions, this book serves as an important resource for analysts interested in this area of complex analysis. Notes, numerous exercises, and a comprehensive up-to-date bibliography provide an accessible entry to anyone with a standard graduate background in real and complex analysis. "The book has a comprehensive and up-to-date bibliography, which is a valuable asset to any researcher in the field. ? Without a question, the book under review is useful for the student contemplating working in the field. I am also happy to notice that the book has something to offer for researchers at all levels, as it is a thorough and illuminating presentation of the current state of the theory." (Janne Gröhn, Mathematical Reviews, March, 2018)