The nation's most extensive and intensive study of the planet Venus and its enviroment occurred during and subsequent to December 1978: Pioneer Venus. This introductory paper to a special issue of the Journal of Geophysical Research devoted to the scientific findings of both the orbiter and multiprobe missions documents the major aspects of the missions and provides the reader with necessary background information to cope intellingently with the myriad of scientific findings found elsewhere in the issue. Specific topics covered include program history, key scientific questions addressed by Pioneer Venus, scientific payloads, science management, spacecraft descriptions, launch and interplanetary cruise features, encounter features, nominal mission descriptions, instrument anomalies, and the orbiter extended mission.