The present review focuses on what is known of basic serotonin physiology in the human body. Here, we describe serotonin biochemistry and metabolism and summarize the results of studies that have contributed significantly to our understanding of serotonin physiology. We report the well-established role of serotonin in cardiovascular, gastrointestinal, and circulatory physiology. Emphasis is placed on the role of serotonin in peripheral physiological systems rather than in the central nervous system. A brief overview is provided on the emerging role of serotonin in novel areas such as bone pathways and glucose uptake. We also report a select few animal studies and animal models that have provided worthwhile contributions to the understanding of serotonin in human physiology. In addition, we summarize the results of large-scale genetic studies on serotonin and serotonin transporter genes, performed in relation to behavioral and mood disorders. J. Cell. Physiol. 217: 301–306, 2008. © 2008 Wiley-Liss, Inc.