Demand for high-protein snack bars as meal replacers and by consumers engaged in sports and dieting has grown significantly in recent years. These products provide healthy alternatives to conventional snacks because of the inclusion of high levels of protein (15–35%, w/w) and other nutritionally beneficial ingredients. Inclusion of high levels of protein in bars may result in adverse quality effects, in particular bar hardening, the degree of which is dependent on the ingredients and process used to produce the bars. Particularly water migration to the protein powder particles is a strong contributor to hardening of protein bars due to subtraction of water from different constituents, which can lead to hardening due to crystallization of carbohydrates. Protein bars containing whey protein isolate as the protein source generally developed less hardness than those containing caseinate. Part of these differences can be related to differences in moisture sorption of the protein powder particles in the water activity region of the protein bars (~0.5–0.7). For understanding protein functionality in bars, it is important to keep in mind that at these water activities, proteins should not be considered as fully hydrated proteins, but as partially hydrated. A further strong contributor to protein ingredient behavior in protein bars is powder particle size and microstructure. A broad particle size distribution and agglomeration were found to be beneficial to reduce development of hardness. Such effects can be attributed to the fact that less dense packing of powder particles is achieved in polydisperse systems, as a result of which hardness is reduced. The use of protein blends can also be beneficial from this perspective. Overall, initial hardness of protein bars can be described initially as a function of volume fraction of suspended powder particles and the efficiency of packing thereof. During storage, water migration and partial hydration and swelling of protein particles further contribute to hardness development. Hence, control of water sorption of powder particles and powder particle size and structure are prime routes to control the development of hardness in protein bars.