Gastrointestinal hormones are chemical messengers that regulate a broad range of physiologic functions. Although primarily expressed within tissues of the gut, these peptide hormones are widely distributed throughout the body and act on multiple target tissues. Furthermore, these regulatory peptides can exist in multiple molecular forms that may bind to multiple cell-surface receptors coupled to one of several possible signal transduction systems leading to diverse biologic responses. With such an expansive field to study, it is not surprising that gut endocrinologists have embraced the new techniques that are emerging from the revolution of molecular biology. Beginning with the first construction of a recombinant DNA molecule by Paul Berg in 1971, molecular biology has developed many new techniques that have been rapidly adopted by gut endocrinologists to enable a more detailed understanding of gastrointestinal function. The merging of these two fields has led to a new area of research, molecular gut endocrinology, or the study of gut physiology and endocrinology at the level of individual molecules (ranging from polypeptide-surface receptors to small-molecule second messengers to DNA sequences). Gut cells are constantly bombarded by numerous hormones, and the tightly regulated physiologic status of each cell is becoming more clearly understood.