Models to describe drying pmesses are necessary for engineering design and optimization. Many resenrch studies in the past century have had the objective of developing mathematical models to describe drying processes in porous solids. This review identifies key characteristics of drying models including controlling process resistances, internal mechanisms of moisture movement, and methods of model cufficient determination, model solution, and model validation. Similarities and differences between previous work are noted, and strategies for future drying model development are discussed.