This chapter provides a brief overview of the endocrine system by describing the different types of hormones and the organs involved in their production. The functions of the main hormones are summarised and the multifaceted relationships between the endocrine system and the nervous systems are discussed. Hormones should not be seen as isolated substances, but as active components of complex pathways with dynamic interactions and signalling mechanisms. A more in-depth understanding of hormonal pathways has led to the emergence of psychoneuroendocrinology, a modern clinical discipline that investigates the reciprocal influences between brain, endocrine system, and psychological processes. Imbalances in hormonal levels can result in pathological changes in both the brain and the body. Specific pathologic responses, such as general adaptation syndrome and allostatic overload, have been described and linked to paradigmatic examples of endocrine imbalances. A neuroendocrinological perspective on psychiatry provides valuable insights about the multiple contributions of hormones to the mechanisms underlying psychopathology.