Fly fishing can be done in fresh or salt water. North Americans usually distinguish freshwater fishing between coldwater species (trout, salmon, steelhead) and warmwater species, notably bass. In Britain, where natural water temperatures vary less, the distinction is between game fishing for trout or salmon and coarse fishing for other species. Techniques for freshwater fly fishing also differ with habitat (lakes, small streams and big rivers.)

In fly fishing, fish are caught by using artificial flies that are cast with a fly rod and a fly line. The fly line (today, almost always coated with plastic) is heavy enough to send the fly to the target. This is one of the main differences between fly fishing and spin or bait fishing; in fly fishing it is the weight of the line that carries the hook through the air, whereas in spin and bait fishing it is the weight of the lure or sinker that gives you casting distance. Artificial flies are of several types, some imitating an insect (either flying or swimming), others a bait fish or crustacean, others attractors are known to attract fish although they look like nothing in nature. Flies can be made either to float or sink, and range in size from a few millimeters to 30 cm long; most are between 1 and 5 cm.

Artificial flies are made by fastening hair, fur, feathers, or other materials, both natural and synthetic, onto a hook. The first flies were tied with natural materials, but synthetic materials are now very popular and prevalent. The flies are tied in sizes, colors and patterns to match local terrestrial and aquatic insects, baitfish, or other prey attractive to the target fish species.

Fly fishing is most renowned as a method for catching trout and salmon, but today it is also used for a wide variety of species including pike, bass, panfish, grayling and carp, as well as marine species, such as redfish, snook, tarpon, bonefish and striped bass. There are many reports of fly anglers taking unintended species such as chub, bream and rudd while fishing for 'main target' species such as trout. There is a growing population of anglers whose aim is to catch as many different species as possible with the fly. Even catfish can be occasionally caught while fly fishing.

Many credit the first recorded use of an artificial fly to the Roman Claudius Aelianus near the end of the 2nd century. He described the practice of Macedonian anglers on the Astraeus River:

In his book Fishing from the Earliest Times, however, William Radcliff (1921) gave the credit to Martial (Marcus Valerius Martialis), born some two hundred years before Aelian, who wrote:

The last word, somewhat indistinct in the original, is either "mosco" (moss) or "musca" (fly) but catching fish with fraudulent moss seems unlikely.[citation needed]

Modern fly fishing is normally said to have originated on the fast, rocky rivers of Scotland and northern England. Other than a few fragmented references, however, little was written on fly fishing until The Treatyse on Fysshynge with an Angle was published (1496) within The Boke of St. Albans attributed to Dame Juliana Berners. The book contains, along with instructions on rod, line and hook making, dressings for different flies to use at different times of the year. The earliest English poetical treatise on Angling by John Dennys, said to have been a fishing companion of Shakespeare, was published in 1613, The Secrets of Angling, of which 6 verses were quoted in the better known book Izaak Walton's Compleat Angler (1653), of which the latter two chapters were actually written by his friend Charles Cotton, and described the fishing in the Derbyshire Wye.

British fly-fishing continued to develop in the 19th Century, with the emergence of fly fishing clubs, along with the appearance of several books on the subject of fly tying and fly fishing techniques. In southern England, dry-fly fishing acquired an elitist reputation as the only acceptable method of fishing the slower, clearer rivers of the south such as the River Test and the other chalk streams concentrated in Hampshire, Surrey, Dorset and Berkshire (see Southern England Chalk Formation for the geological specifics). The weeds found in these rivers tend to grow very close to the surface, and it was felt necessary to develop new techniques that would keep the fly and the line on the surface of the stream. These became the foundation of all later dry-fly developments. However, there was nothing to prevent the successful employment of wet flies on these chalk streams, as George Edward MacKenzie Skues proved with his nymph and wet fly techniques. To the horror of dry-fly purists, Skues later wrote two books, Minor Tactics of the Chalk Stream, and The Way of a Trout with a Fly, which greatly influenced the development of wet fly fishing. In northern England and Scotland, many anglers also favored wet-fly fishing, where the technique was more popular and widely practiced than in southern England. One of Scotland’s leading proponents of the wet fly in the early-to-mid 19th century was W.C. Stewart, who published "The Practical Angler" in 1857.

In Scandinavia and the United States, attitudes toward methods of fly fishing were not nearly as rigidly defined, and both dry- and wet-fly fishing were soon adapted to the conditions of those countries.