This chapter provides a short review of general problems connected with functional properties of proteins including main methods of their measurement. Successful utilization of single cell protein as human food ultimately will depend on developing palatable products from this ingredient. Most foods are hydrated solid systems, and the physicochemical, rheological behavior of proteins and the other constituents of the food is strongly influenced not only by the presence of water but also by the activity of water. Furthermore, dry protein concentrates or isolates must be hydrated when used. For these reasons, the hydration or rehydration properties of food proteins are of great practical interest. Water binding by proteins generally decreases as the temperature is raised because of decreased hydrogen bonding. Denaturation and aggregation occur during heating and the latter may reduce protein surface area and the availability of polar groups for water binding. The viscosity and consistency of protein-containing systems are important functional properties in fluid foods.