The various parameters which interact in the epizootiology of babesiosis and anaplasmosis and which it is necessary to obtain in a survey are analysed and the two diseases compared. Where no data are available from Latin America and the Caribbean, the references from Australia and the United States of America are discussed. The two principal objectives of a study on the epidemiology of babesiosis and anaplasmosis are: to determine the risk of occurrence to understand the relevant factors leading to outbreaks. The first objective is relatively simple to achieve for both diseases by calculating the animal inoculation rate (h) at a determined age and the critical value of h for enzootic stability. The second objective requires exhaustive and complex studies. For babesiosis epidemiology, studies of the tick infection rate, cattle infestation rate, genetic composition of cattle, acaricides, stocking rate, climatic data and other factors are necessary. Anaplasmosis epidemiology cannot be satisfactorily explained by considering it purely as a tick-borne disease. Difficulties to be overcome include the great diversity of haematophagous Diptera in Latin America, and the scarcity of data on the capacity of these insects as anaplasmosis vectors and their feed preferences on cattle.