'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 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 neurons generated in the adult can function and even suggests how they might contribute to cognitive processes.