Adult hippocampal neurogenesis is the lifelong production of new functional neurons from neural stem cells in the adult brain. Production of neurons in the adult brain is limited to two areas. One of these is the subgranular region in the hippocampus dentate gyrus (DG), the other is the subventricular zone of the lateral ventricles. New neurons produced in the hippocampus DG functionally integrate into mature circuits, and contribute to cognition and emotional control, learning and memory functions in particular. It is still unclear how that contribution occurs and a key issue that is being intensively investigated. Hippocampal neurogenesis declines both with age and in neurodegenerative diseases characterized by deterioration of memory. Understanding the contribution of hippocampal neurogenesis to memory function and investigating factors that affect neurogenesis have gained importance for protective or therapeutic approaches against these neurodegenerative diseases. It is suggested that new neurons produced in the DG contribute to spatial and episodic memory function by pattern separation (prevent overlapping of similar information by making separate presentations) through their electrophysiologic properties (being stimulated easier than mature neurons) in the maturation process. Hippocampal neurogenesis is important in episodic and spatial memory formation. In this review we will try to explain how hippocampal neurogenesis occurs, factors that affect the neurogenesis process, and the role of neurogenesis in memory function based on the present literature.