The lichenicolous fungi represent an important ecological group of species that form obligate associations with lichens. They have been studied seriously for over 200 years and the European species are especially well collected. However, collecting in other areas has been far less systematic and many new species await discovery. North American species are especially under-studied and this review is intended to stimulate the collection and study of these species. To encourage this study by lichenologists, we review the sorts of interactions formed by lichenicolous fungi with their lichen hosts, discuss various aspects of host specificity, virulence, chemical ecology, and evolution of lichenicolous fungi and provide a complete taxonomic listing of lichenicolous genera arranged as far as possible into natural groupings. In each section we suggest research topics in need of further study, and provide a listing of significant literature. We hope by calling attention to the largely unexplored biodiversity of lichenicolous fungi, investigators will take up the study of these fascinating organisms.