On December 10, 1974, Helios 1, the first of two nearly identical German-American spacecraft designed to explore interplanetary space in the inner solar system (inside the orbit of earth), was launched into orbit with perihelion 0.3 AU from the sun. Thirteen months later the sister spacecraft, Helios 2, was launched into a similar orbit. To date these are the only spacecraft to reach this proximity to the sun. Each of the Helios spacecraft contained 10 state-of-the-art experiments, variously designed to measure and analyze the solar wind plasma, the interplanetary magnetic field, plasma waves and radio waves, medium- and high-energy solar and galactic cosmic rays, the zodiacal light, interplanetary dust grains, and extra-solar system gamma ray bursts (Helios 2 only). In addition, ground monitoring of Faraday rotations, Doppler shifts, and temporal delays of the Helios radio signals provided information on the magnetic field and electron density within the solar corona close to the solar limb. Communication links with Helios 2 were maintained until March 1980, while Helios 1 continues to provide useful scientific data more than 10 years after launch. This book, written to provide an overview of the scientific, technical, and managerial triumphs of the missions, celebrates the 10th anniversary of successful Helios operations.