Serotonin is an amazing neurotransmitter and hormone that is not only responsible for mood regulation but also plays an important role in regulating various aspects of human physiology and directly affecting human health. Serotonin is key to providing nutrition and oxygen from head to toe through vasoconstriction and hemostasis. The chapters in this book describe in detail not only how the serotonin receptor network interacts with each other but also their role in various pathologies. Disorders of serotonin could be related to structural and/or functional damage to the blood, brain, and bowel. Early identification of these conditions could benefit many diseases such as atherosclerosis, stroke, depression, and irritable bowel syndrome. The effects of serotonin abnormalities extend beyond one’s intellectual capacity. These effects can be felt at any time, from embryonic life to old age. In this book, you will not only find the role of other hormones such as insulin on serotonin but will also learn how serotonin plays an essential role in both plants and animals.