Trapping levels which account for most of the thermoluminescence in natural calcite have been investigated. Results are based on a study of color centers which are shown to be closely associated with the traps involved in thermoluminescence. Optical absorption measurements in a variety of Iceland-spar crystals show that the prominent calcite glow peaks at 350, 500, 600, and \ensuremath{\sim}700\ifmmode^\circ\else\textdegree\fi{}K are accompanied by thermal bleaching of the color centers. Results indicate that the color centers are due to one kind of trapped-hole center and at least four kinds of trapped-electron centers. Recombinations resulting from thermal bleaching of each of these centers excites emission at ${\mathrm{Mn}}^{++}$ impurity ions and produces the observed glow peaks. The mechanism for exciting the ${\mathrm{Mn}}^{++}$ ions appears to be the same nonradiative transfer process which accounts for sensitized luminescence in calcite.