Asteroids (or more properly, minor planets) are small, naturally formed solid bodies that orbit the Sun, are airless, and show no detectable outflow of gas or dust. Although asteroids share many of the characteristics of planets (Sun-centered orbits, seemingly solid bodies), the primary distinction is that they are simply much smaller than the known planets or dwarf planets. Similarly, the distinction between asteroids and comets is also based on their observational qualities rather than any inherent difference in physical properties or composition. This chapter discusses locations, orbits, physical composition, and various other aspects of asteroids.