System behaviour is best described by an appropriate set of non-dimensional combinations of the variables on which it depends. The dimensional analysis technique, elucidated here, allows systematic discovery of such parameter sets. Potential flow theory provides solutions for flows for which viscosity is assumed zero. This theory is developed here to explain and quantify airfoil lift and pitching movement. Unfortunately, viscosity cannot always be ignored as it strongly influences real flows via phenomena such as boundary layer separation. Viscosity's role in laminar and turbulent flow is examined to explain the gap between potential flow theory prediction and experimental observation.