A comprehensive one-dimensional model is developed for heat and mass transfer in each stage of the defrost process. The model describes sublimation, vapor transport, liquid melting and evaporation on a heated vertical surface in each of three distinct stages of the defrost process: diffusion, melting-permeation and dry out. Dimensionless forms of the governing differential equations yield the Lewis, Stefan, and Biot numbers as the key parameters. These parameters for each stage of defrost are slightly different owning to the difference in the dominate heat and mass transfer mechanisms. By analyzing the magnitude of the dimensionless groups, it is possible to determine the relative weight of each term in the governing equation. From such an analysis, the effect of mass transfer due to sublimation in the first and second stages of the defrost process can be neglected. The effects of several limiting cases on the governing equations are evaluated, with simplified equation sets developed for them.