Hormones are chemicals that naturally occur in the body. These chemicals direct many biochemical functions, including energy storage and utilization, maintenance, and growth. Hormones have huge impact on animal behavior. Hormones are released mainly by the endocrine glands. Specific endocrine glands include the pituitary, thyroid, adrenals, parathyroids, gonads (e.g., testes and ovaries). The endocrine glands release very small quantities of hormones into the blood, but these small quantities have a large impact throughout the body. This work is a short review of the most important hormons with an emphasise on sexual hormones and their effect on sex differences and sexual behavior.