We present a first-principles theoretical study of the atomic and electronic structures, and of the zero-temperature phases of gallium. The picture of ±-Ga that emerges is of a metallic molecular crystal with a strong Ga2 covalent bond and weaker intermolecular forces. This picture is supported in detail by the charge density, the electronic structure, and differential bond stretching under pressure. Anomalous features of ±-Ga, such as the Knight shift, anisotropic Fermi-surface effects, and optical absorption find a consistent explanation. Accurate x-ray measurements should reveal the Ga2 covalent bonds. © 1991 The American Physical Society.