Two group-contribution methods have been developed to estimate heat capacities of organic solids at ambient pressure. The power-law (PL) method utilizes an empirical temperature dependence, while the partition function (PF) method is based on the Einstein-Debye partition function for crystals with a modified frequency distribution function. Both methods have a fixed temperature functionality but utilize group contributions to obtain the compound-specific constants in the predictive equations. The training set for the group-contribution correlations consisted of 455 compounds and 7967 heat capacity data points. Both methods can be used for temperatures above 50 K, and they correlate the training set to within approximately 9.4 J(.)mol(-l.)K(-1), corresponding to an average deviation of 6.8% for the PL method and 8.0% for the PF method. The PL method gives better results at lower temperatures, and the PF method at higher temperatures. Tests on the methods' extrapolation capabilities suggest that at 298 K they have comparable accuracy to two currently available methods that are applicable only at 298 K, but the new methods can be used to estimate solid heat capacities over a wide range of temperatures with an average expected accuracy of approximately 13%.