Until recently, historical materials that might shed light on the diverse practices and beliefs surrounding the worship of Japanese native deities, kami, in the medieval period were largely unavailable for academic study. Some still remain hidden in temple treasuries or private collections, waiting to be discovered. ‘Medieval Shinto’ is thus a relatively new field of enquiry, but a fruitful and promising one as well. The current revival of this field is based on a decisive departure from earlier scholarship, and the studies of kami worship are now advancing rapidly. This article will give a brief overview of the major terms, definitions and themes in the study of medieval kami worship. First, we shall see how Japanese kami became associated with Buddhist deities. Secondly, we shall find out how the worship of kami became influenced by Esoteric Buddhism and what types of esoteric kami worship had emerged in medieval Japan. We shall briefly touch upon the rituals dedicated to kami, and how the ideas about native deities became reflected in medieval Japanese arts and literature. Finally, this article will introduce several important books and main areas of research regarding this complex but fascinating subject.