Abstract: The exponential growth in research results on the regulation of adult hippocampal neurogenesis, the life-long addition of new neurons to the hippocampal dentate gyrus, is paralleled by an increasing puzzlement about the potential function of these new cells. But to determine the functional relevance of these new neurons several fundamental problems have to be overcome. Two of them are discussed here. First, it will remain impossible to define the functional contribution the new neurons in the dentate gyrus make to hippocampal function as long as it is unknown how the dentate gyrus itself contributes to hippocampal function. Our hypothesis is that adult hippocampal neurogenesis serves in avoiding a stability-plasticity dilemma between learning new information and preserving old information, by allowing the dentate gyrus to adapt to new input pattern statistics while preserving the ability to process old patterns appropriately. Second, it is still not known, whether in adult neurogenesis the structural alteration follows a specific functional stimulus and serves to consolidate a functional change triggered by that stimulus, or if less specific stimuli of novelty or complexity induce more general structural changes that prophylactically prepare the grounds to better process information in similar novel or more complex situations in the future. Here our experimental findings and theoretical considerations argue for the latter possibility.