Epidemics of malaria seem to occur more frequently in the highlands of eastern Africa. In western Kenya, for example, epidemics have spread from 3 to 15 districts. There is an increasing debate on the possible causes of this phenomenon, but detailed field studies are lacking to test a number of basic hypotheses. In this chapter, we review some of the results from a comparative field study on malaria risk in a highland and a lowland area in western Kenya. Questions that will be addressed in this chapter are: under which conditions may malaria epidemics occur in highland areas and to what extent is the situation in western Kenya comparable to other highland areas in the region?