Abstract: With the rapid urbanization around the globe, the urban heat island (UHI) effects have become an interdisciplinary issue and were received considerable attention during the past 2-3 decades by scientists from different disciplines. In this article, the state of our knowledge on this subject is assessed in order to help guide future research in this rapidly growing field. First, we summarize the latest advances in observing various UHI effects through field experiments and remote sensing technology, and in developing the urban canopy models as well as computational fluid dynamical models. Then, we review recent achievements in understanding the UHI effects with more focuses on the urban boundary layer, their interactions with topographical driven circulations, and the urban surface energy budget. Finally, future prospects for several UHI research avenues are speculated, which include the UHI effects on regional climate, air pollution, precipitation, fog and lightning production, the improvement of urban weather forecasts, and the development of UHI mitigation measures.