Fartlek (Swedish, "speed play", originally in upper case, now generally in lower case), otherwise known as the Swedish natural method or simply the Swedish method, is a middle- and long-distance runner's training approach developed in the late 1930s by Swedish Olympian Gösta Holmér. As more recently described (and therefore accommodating changes in practice and definition that have occurred since the 1930s), fartlek is a "relatively unscientific blending" of continuous training (e.g., long slow distance training), with its steady pace of moderate-high intensity aerobic intensity, and interval training, with its "spacing of [more intense] exercise and rest intervals". Most generally and simply stated, in its widely adapted contemporary forms, fartlek training can be seen simply as alternating periods of faster and slower running, intermixed, often over natural terrain outdoors, including over both "level and hilly terrain".

As such, Holmér's original approach to fartlek training has been widely adapted, and has evolved considerably since its introduction in the early 20th century. While Fartlek training is generally associated with running, it can in principle be incorporated into almost any kind of exercise. The variable intensities and the continuous nature of the exercise stresses both the aerobic and anaerobic parts of the runner's physiology. It differs from traditional interval training in that it is more unstructured; intensities (speeds) and their durations can vary, as the coach and athlete wish.

As stated, although primarily a level surface and standard track training routine in its inception, it has moved far beyond, now including workouts off track, and up grades. An example of its more modern manifestations in the training of serious runners is found in Mona fartlek, named for Australian distance runner Steve Moneghetti, devised by Olympian Chris Wardlaw, which injects speed into a 20-minute session, pairing alternating periods of effort and recovery: 90-seconds on, 90-seconds off (performed twice), then 60-seconds on-then-off, and 30-seconds on-then-off, and 15-seconds on-then-off (each of these performed four times), generally, with intensity (pace) increasing as the effort period shortens, with the specifics determined by coach and athlete.