Understanding epidemiology is central to the development of effective and sustainable strategies for the control of infectious diseases, and epidemiology requires and is related to knowledge of pathogen ecology. This chapter reviews studies on the epidemiology and ecology of cucumber mosaic virus (CMV), an important pathogen with a broad host range, including the following topics: the natural host range of CMV; the occurrence and incidence of CMV infection; CMV population structures in the field; and the occurrence of highly virulent isolates of CMV, their effects, and their population dynamics. The present knowledge regarding CMV ecology and epidemiology fails to explain outstanding features of its biology, such as its extremely broad host range, its ubiquity, and the intrinsic variability of its abundance in plant populations. The use of new molecular and bioinformatics tools in field studies is expected to lead to major advances regarding the major outstanding issues of CMV epidemiology.