The most striking characteristics of the myenteric plexus are the heterogeneity of its neuronal populations and the complexity of its organization. Myenteric neurons greatly differ in their morphological characteristics, projection patterns, and topographical arrangement within the ganglia. The discovery of histochemically distinct types of neurons together with the development of nerve-tracing techniques and specific lesions have allowed a better understanding of the relationships of enteric neurons to specific target tissues. Consequently, these techniques have contributed significantly to our knowledge of the highly ordered organization of the ENS, which represents the anatomical substrate for the neural coordination and integration of the complex functions it subserves. The existence of different types of neurons that probably use different substances as transmitters may reflect on the existence of defined functional roles for each type of neurons. To date, only ACh, NE, and probably 5-HT seem to satisfy all the criteria necessary for establishing a neurotransmitter, although there is increasing evidence that some of the other substances, such as GABA, SP, and VIP, are enteric transmitters or modulators. The functional roles of the different types of neurons in the neural circuitry that regulates gastrointestinal functions remain to be elucidated.