The analysis of the X-ray emission from a sample of 55 bright early-type galaxies shows that hot gaseous coronae are a common and perhaps ubiquitous feature of such systems. The X-ray emission can be explained most naturally as thermal bremsstrahlung from hot gas (kT ≈ 0.5–1.5 keV) which may be accumulated from mass loss during normal stellar evolution. The presence of these coronae shows that matter (109−1010 M⊙) previously thought to be expelled in a galactic wind is instead stored in a hot galactic corona which may be heated and powered by supernova explosions. Perhaps the single most important feature of these coronae is that they provide a unique tracer of the gravitational potential in the outer regions of bright early-type galaxies. In this paper we describe the X-ray properties of these coronae (gas mass, temperature, and extent) and discuss their implications for the presence of massive dark halos around individual early-type galaxies. We find total masses of early-type galaxies up to 5 × 1012 M⊙. We estimate mass-to-light ratios for early-type galaxies and find values up to ∼100 (in solar units), similar to those found for the larger dynamical systems of groups and clusters.