Variational transition state theory with multidimensional tunneling contributions has been used to calculate the rate constants, kinetic isotope effects, and activation energies for 1,2-shifts in methylchlorocarbene, benzylchlorocarbene, cyclopropylfluorocarbene, and cyclopropylchlorocarbene. Calculations have been performed for the rearrangements both in the gas phase and in various solvents. Including solvation effects reduces the calculated activation barrier for each of these reactions. The effects of quantum mechanical tunneling are computed to be significant for the 1,2-hydrogen migrations and to be bigger for hydrogen than for deuterium. Consequently, the deuterium kinetic isotope effects are predicted to be relatively large but to decrease with increasing temperature. In contrast, tunneling is not calculated to play a significant role in either of the halocyclopropylcarbene rearrangements, which both involve the 1,2-shift of a CH2 group. Thus, heavy-atom tunneling is apparently not responsible for the fact that the calculated activation parameters are very different from experiment for cyclopropylfluorocarbene, with the experimental activation enthalpy much smaller than the calculated one and the experimental activation entropy much more negative than the computed value. Possible causes for the large differences between the calculated and measured activation parameters are discussed.