This book illustrates how potential flows enter into the general theory of motions of viscous and viscoelastic fluids. Traditionally, the theory of potential flow is presented as a subject called 'potential flow of an inviscid fluid'; when the fluid is incompressible these fluids are, curiously, said to be 'perfect' or 'ideal'. This type of presentation is widespread; it can be found in every book on fluid mechanics, but it is flawed. It is never necessary and typically not useful to put the viscosity of fluids in potential (irrotational) flow to zero. The dimensionless description of potential flows of fluids with a nonzero viscosity depends on the Reynolds number, and the theory of potential flow of an inviscid fluid can be said to rise as the Reynolds number tends to infinity. The theory given here can be described as the theory of potential flows at finite and even small Reynolds numbers.