Serotonin 5-hydroxytryptamine (5-HT) is an important regulator of morphogenetic activities during early central nervous system development, including cell proliferation, migration, and differentiation as well as synapto-genesis. Serotonergic raphe neurons diffusely project to a variety of brain regions (e.g. cortex, amygdala, hippocampus) and play known roles in integrating emotion, cognition, motor function as well as in food intake, sleep, pain, and sexual activity. The diversity of physiologic functions is due to the fact that 5-HT acts as a master control neurotransmitter within a highly complex system of neural communication mediated by multiple pre- and postsynaptic 5-HT receptors, thus orchestrating the activity and interaction of several other neurotransmitter systems. Since proteins involved in the regulation of central serotonergic activity (e.g. enzymes, receptors, transporter) play pivotal role in brain 5-HT homeostasis, polymorphisms in the regulatory regions of their genes resulting in variation of expression and function are likely to influence complex traits, such as temperament/personality and psychopathology.