Major advances have been made in our understanding of the nervous system in the gastrointestinal tract, the enteric nervous system. Because of its importance, neurogastroenterology is being increasingly recognised in clinical pharmacology. The enteric nervous system is a collection of neurones that can function more or less independently of the central nervous system and controls or modulates motility, exocrine and endocrine secretions, microcirculation and immune and inflammatory processes. Increasing knowledge of the physiology, pathophysiology, and pharmacology of the enteric nervous system will provide a basis for creation of new approaches to the treatment of gastrointestinal disorders. This review is part one of three and will describe the organisation and classification of the enteric nervous system.