Spray drying is a fascinating unit operation that involves several complex stages and presents serious difficulties when trying to model it on a fundamental basis. Difficulties arise from the fact that existing relationships can not take into account all the variables involved in the process. Results derived from the application of reported models give only an order of magnitude estimate of design and processing parameters. Besides, the particles undertaking drying do not have uniform size and shape, and these properties vary along the process, giving place to a very complex multi-phase mixture of gas, solids, liquid and rubbery materials.