Textbook in PDF format This book begins with discussing the fundamentals of Simultaneously Transmitting and Reflecting Surfaces (STARS) from the electromagnetic (EM) and communication perspectives. The basic signal and channel models for employing STARS in wireless communications is introduced as well. Then, the different categories and possible hardware implementation of STARS are highlighted. Next, the authors focus on attention to the STARS-aided wireless communications. The authors provide a comprehensive performance analysis of STARS with the outage probability, diversity gain and power scaling laws. Moreover, the operating protocols and corresponding beamforming design of STARS are discussed under different phase-shift models