The hydrodynamic principles that deal with the mechanics of fluid flow and the derivations are based on three conservation principles: Mass, momentum and energy. In this chapter, these are initially discussed from the viewpoint of classical hydrodynamics and then with reference to their application in open channel flow. The continuity equation ensures the conservation of mass. The specific force equation is based on the momentum principle and calls for force balance. The specific energy equation is based on the energy principle and calls for energy balance. These important principles related to open channel flow are discussed and applications are explained. The additional features of this chapter are the introduction to the boundary layer theory, flow in a curved channel, hydrodynamic drag and lift on a particle and Stokes law. © 2014, Springer-Verlag Berlin Heidelberg.