This book provides a thorough review of multi-mode propagation inside optical waveguides and free space, which is receiving particular attention for its promising applications in communications and sensing. At the heart of the book is the matter of how modes couple and interfere due to engineered or random index fluctuations, forming functional devices. The chapters cover topics such as multi-mode interference, coupled mode theory, and mode generation. Readers discover how a universal coupled mode theory can describe mode propagation, enabling stochastic analysis and avoiding time-consuming simulations. The book also delves into mode division multiplexing systems and digital signal processing (DSP) algorithm-enabled multiple-input multiple-output (MIMO) transmission in multi-mode systems. Researchers in the field of optical communications and for physicists and engineers will find this book to be invaluable. It offers a comprehensive review of multi-mode transmission systems basic physics and applications, making it essential for anyone interested in advancing their understanding of this rapidly expanding field.