The energy cost per unit distance (i. e. the economy of swimming, C) is given by the ratio / v where is the net (above resting) metabolic power and v is the swimming speed. The contribution of the aerobic and anaerobic energy sources to in swimming competitions differs according to the distance covered; it is independent of swimming style, gender or skill and depends essentially upon the duration of the exercise. In swimming, C increases with the speed with a non linear function; for a given speed, C is the lowest for the front crawl, followed by the backstroke, the butterfly and the breaststroke. C is essentially determined by the hydrodynamic resistance (Wd): the higher Wd, the higher C; and by the propelling efficiency (ηP): the higher ηP the lower C. Hence, all factors influencing Wd and/or ηP will result in proportional changes in C. The concepts of economy and efficiency are strictly related; hence, this chapter is also devoted to an analysis of the efficiencies in swimming; a summary of the values reported in the literature is also presented from a “historical point of view”. Last but not least the factors setting performance ( max and C) are briefly reviewed in view of a proper planning of swim training.