Ludwig Prandtl, with his fundamental contributions to hydrodynamics, ae- dynamics, and gas dynamics, greatly in?uenced the development of ?uid - chanics as a whole, and it was his pioneering research in the ?rst half of the last century that founded modern ?uid mechanics. His book Fuhrer durch die Stromungslehre, which appeared in 1942, originated from previous pub- cations in 1913, Lehre von der Flussigkeit und Gasbewegung, and 1931, Abriss der Stromungslehre. The title Fuhrer durch die Stromungslehre, or Essentials of Fluid Mechanics, is an indication of Prandtl's intentions to guide the reader on a carefully thought-out path through the di?erent areas of ?uid mech- ics. On his way, the author advances intuitively to the core of the physical problem, without extensive mathematical derivations. The description of the fundamental physical phenomena and concepts of ?uid mechanics that are needed to derive the simpli?ed models has priority over a formal treatment of the methods. This is in keeping with the spirit of Prandtl's research work. The ?rst edition of Prandtl's Fuhrer durch die Stromungslehre was the only book on ?uid mechanics of its time and, even today, counts as one of the most important books in this area. After Prandtl's death, his students Klaus Oswatitsch and Karl Wieghardt undertook to continue his work, and to add new ?ndings in ?uid mechanics in the same clear manner of presentation.