To conduct any sports activity effectively, the technique of each sport must be known. The "effective" technique is not an abstract and general concept, but it is the way of conducting movements so that the forces exerted, e.g., for the propulsion of the body, are exerted in the most effective possible way and with the least possible expenditure of energy. In swimming, as water, due to its greater density compared to air, "maximizes" the "mistakes" and resistances that hinder the movements and propulsion of swimmers, the "correct" technique is a much more decisive factor for achieving high performance and avoiding injuries, compared to all other sports. Although each swimmer has their individual peculiarities and consequently there is no "one perfect" technique for everyone, there are some "rules" and "criteria" for the "correct" and "effective" technique of swimming styles. This is precisely the subject of this book, where an attempt is made to present and analyze the technique of the four swimming styles (freestyle, backstroke, butterfly, breaststroke) and the corresponding turns and starts, emphasizing the "image," through many diagrams, and understanding the "nature" of resistance forces and propulsive forces that develop during movement in water. In the daily effort to "perfect" the technique, it is particularly important for swimmers to "visualize" their movements and acquire the "feel" of the water. By understanding the properties of water, they can exploit them to the maximum possible degree, achieving not only better competitive performance but also simultaneously preventing potential injuries due to poor technique. Gourgoulis Vasileios Professor T.E.F.A.A. - S.E.F.A.A - D.P.TH.