The scavenging process in the cylinder of a two-stroke-cycle internal-combustion-engines is the most important factor controlling the engine efficiency and performance characteristics. A reliable description model of the gas exchange process may be a powerful tool to the engine designer. To benefit the most, the designer should select the best model for his specific purpose. This paper reviews and assesses many scavenging models which have been proposed in the literature for the past three decades. The scavenging models were divided into three categories: One-phase, Multi-zone and Hydrodynamics models and each model has been analyzed for its basic assumptions and prediction ability.