Adult hippocampal neurogenesis has changed our view on the adult mammalian brain by adding a new kind of neuroplasticity with strong implications for cognition and behaviour of older mammals. Because the new neurons are found in the hippocampus, a structure well known to be the gateway to memory, adult neurogenesis appears to be of especially high relevance for humans. The integration of new neurons into existing and wellworking networks depends on an intimate regulation of the divisions the putative stem cells undergo in the subgranular zone, the expansion of the precursor cell pool, the processes of maturation and differentiation, and, finally, the appropriate functional integration into the network of the dentate gyrus. While much progress has been made in the last decade on the genetic and molecular underpinnings of adult neurogenesis, as well as how these processes are regulated in the context of different 'lifestyles', its functional contribution remains unclear. From a clinical perspective, however, all the attention adult neurogenesis deserves depends on the specific functional contribution of the new neurons to hippocampal function. Only recently, a coherent picture is about to begin to emerge what the new neurons might be good for. Importantly, results from studies testing rodents in spatial learning paradigms suggest a similar role for adult neurogenesis in human spatial navigation. This chapter reviews the findings on the functional relevance of adult neurogenesis along with a recently published attempt to integrate the data into a common theoretical framework. Beside experimental data, results and implications from studies based on theoretical models regarding the function of the new neurons are considered.