Penguins are the best-adapted birds to wing-propelled diving and swimming. In water the weight of their perfectly streamlined body is balanced by buoyancy. Hence the strong ‘flight’ apparatus is used only for thrust production. Similarly to flying birds penguins flap their wings. But, contrary to fast flight in air, thrust is produced under water during the up-stroke as well as during the down-stroke. Corresponding wing profiles are developed. Since the wings are active in a permanently extended position, the shoulder joint is simplified. The examination of its structure and kinematics revealed a new type of articulation. The ‘rotation’ of the head of the humerus in the shoulder joint can be described as a two-axis-eccentric motion conducted by the ligamentum acrocoracohumerale and an articulation surface of the caput humeri shaped like a logarithmic spiral. This model led to new interpretations of the function of the muscle system. Besides the preponderant thrust-producing (flapping) muscles, working mainly isotonically, muscles can be distinguished which are managing the transfer to the body of the forces produced, thereby operating isometrically. Another group of muscles must control the position of the humerus, adjusting the hydrodynamic angle of attack corresponding to respective flow conditions. In kinematic studies conducted on live penguins, different modes of wing action were observed. The functional anatomical demands of the balance of forces and momentum are discussed with respect to the non-steady flow around the wings.