In richness of scientific return, the Apollo 15 voyage to the plains at Hadley compares with voyages of Darwin's H.M.S. Beagle, and those of the Endeavour and Resolution. Just as those epic ocean voyages set the stage for a revolution in the biological sciences and exploration generally, so also the flight of Falcon and Endeavor did the same in planetary and Earth sciences and will guide the course of future explorations. The boundary achievements of Apollo 15 cannot now be established. As an author of a following paper points out, the mission was not finished at splashdown in the Pacific, nor later with painstaking analysis in scores of laboratories of the samples and cores brought back, nor with careful study of the photographic imagery and instrument traces returned home. For the distinctive fact is that the mission is not yet over. Data still flow in daily from the isotope-powered station emplaced on the plain at Hadley, and from the Moon-encircling scientific satellite left in orbit. This data flow is of exceptional value because it now affords, for the first time, a triangulation of lunar events perceived by the three physically separated scientific stations that man has left on the Moon. This volume is the first, though assuredly not the final, effort to assemble a comprehensive accounting of the scientific knowledge so far acquired through this remarkable mission. The Apollo 15 mission was the first of the Apollo missions to utilize the full capability of a complex set of spacecraft and launch vehicles, the design, development, and construction of which have occupied the major efforts of the U.S. space program for the last decade. The reliability and capability of the Apollo spacecraft, launch vehicles, and ancillary equipment such as space suits were extensively tested and demonstrated in the preceding Apollo missions. These missions also provided the necessary experience in orbital maneuvering and extravehicular activity that enabled the Apollo 15 crew and Mission Control Center personnel to undertake a mission that was defined almost entirely in terms of its exploratory and scientific objectives. The scope of the Apollo 15 mission differed from that of previous missions in three distinct ways: (1) the command-service module carried a diverse set of experiments aimed at the study of the lunar surface from orbit, (2) the lunar module carried to the surface an electrically powered vehicle that extended the exploration range on the lunar surface by more than a factor of 5 over that of previous missions, and (3) the stay time on the lunar surface was extended to twice that of previous landings. The full utilization of this enhanced capability provided results that furnish many new insights into lunar history and structure. Perhaps most important of all, this mission provided results that give a meaningful overall picture of the Moon. The scientific endeavors of the Apollo 15 mission can be divided into three distinct kinds of activities: (1) the orbital experiments, (2) the package of lunar-surface experiments, and (3) the surface sampling and observation.