This guide is an attempt to consolidate all information pertinent to the taxonomy of the genus Didymium, including uniform species descriptions and a key for all of the species, and to make this information available to interested persons in an open access journal. Didymium is a genus, in which over ninety accepted species have been described, that is defined by the presence of crystalline lime granules occurring on the peridium but not the capillitium. The number of different species and the morphological variability within many of them has produced a situation where it is sometimes difficult to identify a particular specimen. Thus, this article is an attempt to provide guidance in the identification of the sessile species.