In the fields of image and audio processing, deep learning technology has won the title of “king”, but it is still in its infancy in the field of recommendation. All of the five authors of this book have worked for Tencent, they were engaged in the practice and research related to recommendation system in different departments. This book summarizes the authors’ past working experiences to guide the practitioners who intend to transfer to the domain of recommendation system or the ordinary fans of recommendation system. From easy to difficult issues, the content of this book transits from the traditional recommendation algorithm to the recently emerging deep learning technology. This book introduces deep learning recommendation techniques that have been proved to be effective in practice, including Word2Vec, Wide & Deep, DeepFM and GAN, and gives the related practice code. In addition to explaining the implementation of the recommendation system at the algorithmic level, this book also elaborates how to build the recommendation system at the engineering level.