This paper reviews the different levels and scales of mathematical modelling that can be applied to spray dryers. These scales range from the whole dryer level (coarsest scale), represented by a well-mixed assumption, through plug-flow reactor approximations (and mixed well-mixed/plug-flow views, a finer scale), to the finest scale, computational fluid dynamics (CFD). All levels have some value, and the approach is suggested here of applying the coarsest scale first, where well-mixed approaches can give some useful information about the occurrence of wall deposition in short-form dryers. Parallel-flow approaches give some insights into trends in crystallinity and direct wall deposition from an atomizer. CFD, as a means of studying the finest scale phenomena, has been heavily used recently for studying the wall deposition process and for suggesting new designs for dryers. (C) 2009 Elsevier Ltd. All rights reserved.