Testing of cellulases on real biomass samples is required to do a true assessment of their efficacy for biomass degradation. Cellulase enzymes belong to a number of different glycosyl hydrolase families, all with different activity, specificity and modes of action. The concerted and synergistic action of these different cellulases determines the efficacy for plant cell wall deconstruction and cellulose hydrolysis. However, the plant cell wall of lignocellulosic materials is a very complex matrix and the efficacy of a cellulase preparation to degrade lignocellulosic materials is influenced by many factors. In this chapter, two protocols for testing efficacy of cellulases on pretreated biomass samples are described. The first protocol describes a small-scale setup employing low solids concentration that easily enables the testing of a larger number of samples. The second protocol describes a method for testing the efficacy of cellulases at conditions more closely resembling industrial conditions, i.e., high solids concentrations. Both protocols can be used to test the cellulases under a variety of substrate types, substrate concentrations, enzyme loadings and process conditions. The protocols can also be used to evaluate different feedstocks.