Neurotrophins are a family of growth factors that are capable of stimulating the survival, development and function of neurons. Neurotrophins act by preventing the associated neuron from initiating programmed cell death, and thus allowing the neurons to survive. Neurotrophins also help to stimulate and control neurogenesis, a process that new neurons develop from neural stem cells. The term “Neurotrophin” refers to four structurally related neurotrophic factors: nerve growth factor (NGF), brain-derived neurotrophic factor (BDNF), neurotrophin-3 (NT-3), and neurotrophin-4 (NT-4). Novel neurotrophin-1 (NNT1) is another neurotrophic factor, but it is structurally unrelated to NGF, BDNF, NT-3 and NT-4. Neurotrophins act through two classes of receptors: p75 and the Trk family of receptor tyrosine kinases (TrkA, TrkB and TrkC). Each neurotrophin has been shown to activate one or more of the three Trks. p75 neurotrophin receptor (p75NTR), a member of the tumour necrosis factor receptor superfamily, is also activated by each neurotrophin. Neurotrophins stimulate signaling pathways controlled through Ras, phosphatidyl inositol-3 (PI3)-kinase and phospholipase C-gamma1. Neurotrophins control the survival and development of neurons. Neurotrophins also play important roles in regulating axon growth, dendrite cell growth and pruning and the expression of proteins. Aberrant control of neurotrophins and their receptors lead to many diseases. For example, the neurotrophin receptor p75 is related to Alzheimer’s disease.