Three models of masking additivity are reviewed, which are referred to as the high-compression model [M. J. Penner, J. Acoust. Soc. Am. 67, 608-616 (1980); M. J. Penner and R. M. Shiffrin, J. Acoust. Soc. Am. 67, 617-627 (1980)], the power-law model [R. A. Lutfi, J. Acoust. Soc. Am. 73, 262-267 (1983); 80, 422-428 (1986)], and the modified power-law model with compressed internal noise [Humes et al., J. Acoust. Soc. Am. 83, 188-202 (1988)]. While the high-compression model was derived from data for two or more nonsimultaneous maskers and the power-law model was derived from data for two or more simultaneous maskers, the modified power-law model can be applied to both cases. The modified power-law model assumes that the threshold in quiet is equivalent to a masked threshold resulting from an internal noise that is continually present. Additional assumptions concern the interaction of two maskers prior to the addition of the masking effects. Most of the data on the additivity of masking are well described by the modified power-law model, regardless of the nature of the maskers. Thus the model provides a good description of data for combined simultaneous maskers and combined nonsimultaneous maskers, a task heretofore requiring the use of at least two separate and independently developed models.