Brain plasticity refers to the brain's ability to change structure and/or function during maturation, learning, environmental challenges, or disease. Multiple and dissociable plastic changes in the adult brain involve many different levels of organization, ranging from molecules to systems, with changes in neural elements occurring hand-in-hand with changes in supportive tissue elements, such as glia cells and blood vessels. There is now substantial evidence indicating that new functional neurons are constitutively generated from endogenous pools of neural stem cells in restricted areas of the mammalian brain, throughout life. So, in addition to all the other known structural changes, entire new neurons can be added to the existing network circuitry. This addition of newborn neurons provides the brain with another tool for tinkering with the morphology of its own functional circuitry. Although the ongoing neurogenesis and migration have been extensively documented in non-mammalian species, its characteristics in mammals have just been revealed and thus several questions remain yet unanswered. "Is adult neurogenesis an atavism, an empty-running leftover from evolution? What is adult neurogenesis good for and how does the brain 'know' that more neurons are needed? How is this functional demand translated into signals a precursor cell can detect? "[corrected].Adult neurogenesis may represent an adaptive response to challenges imposed by an environment and/or internal state of the animal. To ensure this function, the production, migration, and survival of newborn neurons must be tightly controlled. We attempt to address some of these questions here, using the olfactory bulb as a model system.