During the last two decades, neurogenesis in the adult mammalian brain has been extensively investigated. Studies have indicated that two brain regions, the subgranular zone of the hippocampal dentate gyrus and the subventricular zone of the lateral ventricle, possess the most active progenitor cells that are capable of generating neurons throughout the lifespan of human beings. Adult hippocampal neurogenesis is the focus of this review. We intend to discuss the changes in the hippocampal neurogenesis caused by pathologic brain insults such as brain ischemia, traumatic brain injury, epileptic seizures, neurodegenerative disorder, and psychiatric diseases. Further, we discuss the stimulatory and inhibitory actions on adult hippocampal neurogenesis by biochemicals and pharmacological agents, including antidepressants, antipsychotics, agonists and antagonists of glutamate and GABA, adrenal corticoids, gonadal hormones, growth factors such as insulin-like growth factor I, erythropoietin, and drugs of abuse, including nicotine, alcohol, opiates, cocaine, methamphetamine, and 3,4-methylenedioxymethamphetamine (“ecstasy”).