The majority of the research conducted over the past five decades has concentrated on the description of frost formation and growth. The prevailing ambient environment greatly influences frost morphology. Several models have been proposed to describe time-variant physical properties and growth of the frost layer, and several researchers have developed frosted fin models to predict the thermal performance of heat exchangers. Experiments have visualized the growth of frost on simple and finned surfaces, as well as, quantified the degradation of the system performance and efficiency under frosted conditions. Recently, studies have been completed to experimentally determine the heat load imposed on the refrigeration system during defrosting and recovery cycles. There have been relatively few models proposed to predict the heat transfer in defrost, with very little analysis of mass transfer. In this chapter we examine several relevant modeling efforts on frost formation and defrost.