Some exploratory enterprises start with fanfare and end with a quiet burial; some start with hardly a notice, yet end up significantly advancing mankind's knowledge. The Interplanetary Pioneers more closely fit the latter description. When the National Aeronautics and Space Administration started the program a decade ago it received little public attention. Yet the four spacecraft, designated Pioneers 6, 7, 8, and 9, have faithfully lived up to their name as defined by Webster, "to discover or explore in advance of others." These pioneering spacecraft were the first to systematically orbit the Sun at widely separated points in space, collecting information on conditions far from the Earth's disturbing influence. From them we have learned much about space, the solar wind, and the fluctuating bursts of cosmic radiation of both solar and galactic origin. These Pioneers have proven to be superbly reliable scientific explorers, sending back information far in excess of their design lifetimes over a period that covers much of the solar cycle. This publication attempts to assemble a full accounting of this remarkable program.