‘Vascular-streak dieback’ (VSD) was the name given to a devastating disease of cacao, first described in Papua New Guinea in the 1960s and later found to be caused by a previously undescribed tullasneloid basidiomycete, Oncobasidium theobromae (now Ceratobasidium theobromae). This chapter describes the symptoms of VSD and the biology of the pathogen, the disease’s epidemiology and methods of management, as well as including a detailed case study showing the importance of field studies of the disease. The chapter anticipates future trends for research into this disease and suggests further reading on the subject.