Revision with unchanged content. Future wireless systems are expected to reliably communicate broadband data in high mobility environments. Such communication systems encounter fading channels whose response exhibits significant variations across both time and frequency. In this book, we study communications over such time- and frequency-selective fast-fading channels, focusing on the noncoherent scenario for which the channel’s realization is a priori unavailable at both the transmitter and receiver. In particular, we analyze the fundamental limits on reliable information transfer rates and design pilot-aided transmissions to achieve the promised rates. Our results provide insights on how the rate of time-frequency channel variation influences the achievable information rate as well as the pilot design. The book is intended for engineers and academics involved in the research and design of mobile broadband communication systems.