This book summarizes the latest findings on the functions of microRNAs in the regulation of plant development and responses to the surrounding environment. MicroRNAs are an important class of molecules that can be found in diverse groups of organisms, including plants and animals, and the investigation of their roles is a highly dynamic and “hot” research topic. The respective chapters address four main aspects, namely: microRNA investigation and annotation, the regulatory roles of microRNAs in various developmental processes, in response to abiotic factors, and in the context of biotic stress response regulation. Systematically reviewing the most important findings in this field, the book offers an essential guide for undergraduate and graduate students, teachers, and plant science researchers. Due to the potential applications of microRNAs in crop breeding and plant protection, it also represents a valuable resource for scientists in academia and the private sector alike.