In neuroethology, the nervous system and behavior are analyzed in the context of the animal's natural habitat and evolutionary history. For the last 30 years the influence of genetics on neuroethology has steadily grown, particularly in Drosophila. Genetic variants reveal new properties of neurons; they help to dissect neuronal circuits and complex behavioral systems; genetics provides new methods to visualize certain brain structures and to assign behavioural functions to them; and, finally, genetic variants can be used to test ecological models. While single-gene mutations can hve highly specific behavioral effects, molecular analysis of the corresponding genes reveals that the latter normally have a much broader functional scope. The 'graininess' of a functional model of the brain, therefore, is defined by the independent regulatory units of the genes rather than by the genes themselves.