This book is comprised of different paper contributions published in "Remote Sensing", under the Special Issue "Radar Imaging in Challenging Scenarios from Smart and Flexible Platforms," for which we served as guest editors. According to the general objective of the Special Issue, the papers included provide a good survey of newborn imaging radar systems, such as synthetic aperture radar (SAR) and ground penetrating radar (GPR), which are designed to operate in challenging scenarios. The latter includes the cases of forward-looking radar or lunar penetrating radar and smart and flexible platforms, such as small airplanes, drones or helicopters. The radar systems described in the book cover a wide range of the microwave spectrum, starting from the VHF band up to the X-band. In addition to the description of newborn systems, the Special Issue includes an important survey of data processing techniques aimed at achieving accurate radar imaging under complex acquisition geometries and/or monitoring scenarios. We thank the Editorial Office of Remote Sensing and all the authors and reviewers for their professional and hard work. Without their excellent contributions, the Special Issue, and thus this book, would not have been possible.