Abstract: "Function" is the key criterion for determining whether adult neurogenesis - be it endogenous, induced, or after transplantation - is successful and has truly generated new nerve cells. Function, however, is an elusive and problematic term. A satisfying statement of function will require evaluation on the three conceptual levels of cells, networks and systems - and potentially even beyond, on the level of psychology. Neuronal development is a lengthy process, a fact that must be considered when neuronal development has to be considered when judging causes and consequences in experiments that address function and function-dependent regulation of adult neurogenesis. Nevertheless, the information that has been obtained and published so far provides ample evidence that adult-generated neurons can function and even suggests how they might contribute to cognitive processes.