The term mucormycosis refers to a group of highly lethal angioinvasive fungal infections, mostly in immunocompromised hosts, caused by members of the order Mucorales, which include various species of the genera Rhizopus, Lichtheimia (formerly Absidia), Mucor, and Saksenaea. Other genera of Mucorales have also been implicated in human disease including Cunninghamella, Apophysomyces, and Rhizomucor. Most infections (50%–65% of total) are caused by Rhizopus species. Classification is in flux because of extensive molecular phylogenetic analysis that has been ongoing for more than 10 years. Note that it is incorrect to use the term mucormycosis to refer only to infections caused by species of the genus Mucor, which are only a small minority of the total number of cases. Rather, mucormycosis refers to infection by any of the organisms within the seven genera of the order Mucorales as noted. The term entomophthoramycosis refers to infections by members of the separate genera Conidiobolus and Basidiobolus. These organisms generally occur in tropics and cause chronic subcutaneous infection mostly in immunocompeten hosts. Only rarely (less than 15 reported cases) have they ever caused clinical syndromes overlapping with mucormycosis. For the remainder of this chapter the term mucormycosis will be used to refer to the range of clinical infections caused by organisms in the seven genera of the order Mucorales. In recent usage this term is generally preferred over the term zygomycosis, because of elimination of Zycomycetes from the taxonomic structure and the very different clinical syndrome as compared to entomophthoramycosis with virtually no clinical overlap.