This chapter focuses on flow and viscosity, including necessary definitions, flow equations and models, a description of the thickening abilities of hydrocolloids and their use as viscosity formers in foods, time dependency of hydrocolloid solutions, fluid gels, and a demonstration of the use and performance of hydrocolloids to control viscosity of foods in recipes for Italian and French dressings that include typical hydrocolloids. The tendency of a fluid to flow effortlessly or with difficulty has unlimited practical importance. The kinematic viscosity is defined as the absolute viscosity of a liquid divided by its density at the same temperature. Apparent viscosity is the viscosity of a non-Newtonian fluid expressed as though it were a Newtonian fluid. The selection of hydrocolloid is dependent on its functional features, availability and obviously, price. Hydrocolloids are used in food processing to improve thickening, gelling, structuring, stabilization, encapsulation and water binding.