The 2×2 reconstruction of indium on the Si(100) 2×1 surface has been studied using scanning tunneling microscopy. For depositions below 150 °C, In lines up in rows perpendicular to the underlying Si dimer rows, with a two unit cell periodicity along each In row. In the 2×2 structure, these rows are spaced two unit cells apart. We investigate the evolution of the In(2×2) reconstruction from isolated rows at lowest coverage, to islands of 2×3 and 2×2 structure, to a surface entirely covered by 2×2 at 0.5 monolayers of In. The role of step edges in nucleating areas of In growth is also examined.