The ionization processes which occur in the collisions of atoms at thermal energies are reviewed. The various experimental methods used to study ionization are discussed; these methods include spectroscopy of decaying plasmas, particle-beam methods for studying particle collisions, Penning electron spectroscopy, massspectrometric detection of reaction products, and methods for selectively exciting the atoms involved in collisions. Information on the cross sections and rate constants of Penning processes, associative ionization, and ionization involving a resonantly excited atom is reported. Results are reported on the spectra of the electrons freed in these ionization processes and on the probabilities for various particular reactions. Theoretical models for ionization in low-energy atomic collisions are discussed.