Geometries, frequency factors, barriers, and reaction enthalpies have been calculated at a variety of levels of theory for methyl radical addition to CH2 = CH2, CH2 = CHCH3, CH equivalent to CH and CH equivalent to CCH3, with a view to selecting reliable computational procedures for studying radical addition to carbon-carbon double and triple bonds. Reaction rates for both the addition and reverse (beta-scission) reactions were also calculated using various transition-state-theory-based procedures, applied at a number of theoretical levels. In general it was found that the geometries, frequency factors and temperature corrections are relatively insensitive to the level of theory, but barriers and reaction enthalpies require a careful choice of theoretical level. Nonetheless, suitable lower-cost alternatives to the high-level W1 results are provided by G3X-RAD and G3(MP2)-RAD. Although errors are somewhat increased, the RMP2/6-311 + G(3df,2p) (for addition to alkenes) or B3-LYP/6-311 + G(3df,2p) (for addition to alkynes) levels of theory also provide a reasonable approximation to the high-level methods. The CBS-QB3 procedure also produces very good reaction enthalpies, but shows a systematic error in the reaction barriers. It appears that the correction for spin contamination in the addition transition structures may be overestimated in standard CBS-QB3 and better results are obtained if the spin-correction term is omitted (U-CBS-QB3).