This textbook covers topics on theoretical analyses of practical low-latency and ultra-reliable communication systems, which is the basis of 5G, B5G and 6G communication systems. The book provides a comprehensive summary of theoretical results for low-latency communication systems, with proofs of main results. Both the information theoretical fundamentals and wireless communications applications with fading are presented. The book is geared towards graduate/senior undergraduate students, helping them to better understand the theoretical foundations and analytical tools of practical communication systems and guide the engineers to find close to optimal solutions in practical communication projects. The book features a suite of pedagogical materials including examples after theorems and definitions, numerical examples to illustrate theoretical results, diagrams to introduce system models, and end of chapter homework problems.