The development of endoscopic ultrasound (EUS) since its introduction in the early 1980s has added greatly to the quality of imaging of the gastrointestinal tract. EUS is probably the investigation of choice for local staging of several gastrointestinal tumors and evaluation of submucosal masses. In addition to well-established indications, newer applications of EUS are emerging. For example, EUS is proving useful in evaluation of other pathologies such as chronic pancreatitis and choledocholithiasis. However, the applications of EUS are no longer limited to the gastrointestinal system, and recent studies have suggested that it has a significant role to play in the staging of nongastrointestinal tumors, such as non–small cell lung cancer. EUS has progressed from being a purely imaging modality to one that can provide a tissue diagnosis by fine needle aspiration (FNA) and deliver therapy (interventional EUS). Indeed, EUS-guided FNA should now be regarded as a routine extension of EUS. The ability to obtain tissue under EUS has made its acceptance more widespread. The field of interventional EUS is one that is in its relative infancy, but many potential applications are under investigation, and there is real promise with several of these applications. It is likely, as with the development of interventional endoscopic techniques, that many of these procedures will be accepted as standard in the future. There continue to be technological advances in the instrumentation used, such as miniprobes and thinner video echoendoscopes. Because the impact of EUS on patient care is increasingly documented in terms of both improved diagnostic accuracy and reduction in costs, the clinical applications of this tool will be difficult to ignore. After years of being limited mainly to academic centers, EUS is finally being performed more widely, but there are training issues that remain to be addressed. This review describes the established and more tentative indications for EUS (Table 1) and covers the applications of EUS in FNA and as an interventional tool. Potential future applications are also discussed. Instrumentation