Since 1944 increasing evidence has been emerging that the adult human brain harbours progenitor cells with the potential to produce neuroblasts. However, it was not until 1998 that this fact was confirmed in the adult human brain. With the purpose of human neurogenesis being hotly debated, many research groups have focussed on the effect of neurodegenerative diseases in the brain to determine the strength of the endogenous regenerative response. Although most of the human studies have focussed on the hippocampus, there is a groundswell of evidence that there is greater plasticity in the subventricular zone and in the ventriculo-olfactory neurogenic system. In this review, we present the evidence for increased or decreased plasticity and neurogenesis in different diseases and with different drug treatments in the adult human brain. Whilst there is a paucity of studies on human neurogenesis, there are sufficient to draw some conclusions about the potential of plasticity in the human brain.