This volume of Astrophysical Data deals with Planets and Stars; a second volume, Part II, will give data for Galaxies and the Universe. They both pro vide basic data for use by all scientists, from the amateur astronomer to the professional astrophysicist. In this first volume, we not only provide physical parameters of planets, stars and their environment, but we also provide the celestial coordinates required to observe them. Here we use c.g.s. units, for they are the most commonly used in astron omy and astrophysics; but our volume begins with astronomical and physical constants and the conversion factors needed for other units. The next section concerns the planets and their satellites; it singles out the Earth and Moon for special treatment. Spacecraft rendezvous with the planets and satellites have led to improved values for their atmospheric compositions, orbital parameters, magnetic fields, masses, radii, rotation periods, and surface pressures and temperatures. This section also contains data for the asteroids, comets and their debris. We then discuss everyday stars, beginning with the Sun, and continuing with basic stellar data, the brightest stars and nearby stars. Special categories of stars, such as the Wolf-Rayet stars, magnetic stars, flare stars, and RS CVn binary stars, are included.