Ultra-Reliable Low-Latency Communications (URLLC) was introduced into 5G networks to facilitate machine to machine communication for such applications as the Internet of Things. But designing URLLC systems, with disjointed treatment of the topic in the literature, has proven challenging. In this work, the authors present a comprehensive coverage of the URLLC including the motivation, theory, practical enablers and future evolution. The unified level of details provides a balanced coverage between its fundamental communication- and information theoretic background and its practical enablers, including 5G system design aspects. The authors conclude by offering an outlook on URLLC evolution in the sixth-generation (6G) era towards dependable and resilient wireless communications. This is the first book to give the reader a complete, yet concise, introduction to the theoretical and application oriented aspects of a topic at the core of both 5G and 6G wireless communication systems. As such, it is essential reading for designers and students of such systems.