This is the leading textbook on the market for graduate level fluid mechanics courses covering viscous and non-viscous flow. Incompressible flow is a required course in preparation for subsequent courses on turbulence and stability. The fourth edition retains the format and philosophy of the earlier editions which has been called "the most teachable book on the market." The presentation starts with basic principles followed with a patient development of the mathematics and physics leading to theories of fluids supported with examples and problem exercises.  For this edition new content will be added on advances in the theory of turbulent shear flows that are not in current textbooks.  A number of older more historical discussions and examples will be reduced in size but left in the text as reference points.  For the first time, the author will incorporate  his own courses test questions and PhD exam questions as homework problems.  A solutions manual will also be available.