In the last decade, the radar world is assisting to a sort of new revolution, comparable to that caused by the introduction of adaptivity in the 70s. Continuing advances in device technologies combined with adaptive processing present rich opportunities for new sensing methodologies and new challenges in signal processing. Complex and integrated systems, waveform diversity, multi-mission and multi-mode operation systems are the new paradigms in the radar field theory and technology. In this context, emerging signal processing theories and methods play a very important role in boosting research and improving performance in radar detection, estimation and tracking, particularly in non-standard conditions, that is, in presence of non-Gaussian disturbance, outliers and interferences, or when only few data are available at the receiver, with applications to many radar systems, spanning from weather radar to scatterometers, from through-the-wall to automotive systems, from active to passive systems. This special issue received more than 80 high-quality submissions and through an extensive peer-review process this final version collects 26 excellent papers touching on a wide variety of topics, including waveform optimization, radar cognition, robust detection and estimation, sparse signal recovery and compressive sensing techniques, being naturally interdisciplinary and involving contributions from experts in the areas of signal processing, array processing, radar systems. The papers are briefly summarized.