Secretin, gastrin and cholecystokinin were the first discovered gut hormones. Today we recognize more than 30 gut hormone genes and a multitude of bioactive peptides, which make the gut the largest endocrine organ in the body. Due to structural homologies gut peptide hormones/growth factors have been divided into separate families. It has been emphasized that those peptides are widely distributed, but have a specific expression in different cell types. The intestine can also be regarded as a sensory organ operating via neurons, endocrine cells and immune cells with gut peptides as signalling substances. Expression studies of peptide receptors in gut neuroendocrine tumours in combination with tailored peptide analogs have been helpful in developing new diagnostic and therapeutic strategies. New fields of research will relate to gut peptides associated with deficiency diseases and as potential growth factors in malignancies. Enterochromaffin cells, interspersed throughout the entire gastrointestinal mucosa, form the largest endocrine cell system. The physiological role of hormonal messengers, peptide receptors and amine transporters is currently under investigation as well as their potential involvement in disease, e.g. the secretory diarrhea associated with midgut carcinoid tumours.