Line is the general term for fishing line, which can come in many variations, all similarly designed to connect an angler to terminal tackle or lures and ultimately (hopefully), fish. Fishing lines were originally held by hand and thrown into the water. Eventually, we invented fishing tools to help cast, control and retrieve our lines. Most sportfishing lines are wound onto fishing reels attached to fishing rods which cast the line out and guide the line back onto the reel. The end of the line cast or released into the water can have lures and baits attached to attract and hook fish. Early lines were made of natural materials such as catgut and silk. Modern fishing lines are made from synthetic materials like nylon, Dacron, polyethylene and fluoropolymer PVDF, and have become highly specialized for use with modern fishing tackle.

 Line can serve many purposes for an angler. Some lines are simply used as filler or backing on larger reels to fill up a spool to aid with casting distance and retrieval speed, or to provide additional length behind specialized lines such as fly lines. Backing can be relatively inexpensive and made of materials that are completely different than the mainline used for the actual cast and retrieve. Mainline is the fishing line typically found as the largest amount on a reel, which may or may not attach directly to the reel and lure, but is always deployed and retrieved. Lines can be colored to aid in better visibility for the angler to manage the line, while other lines are clear or colored in a way to make them invisible to fish. The strength of a line, or test, is measured at the breaking point of the line when pulled in opposing directions. The weight of a fish will not necessarily determine the test strength of line needed, as the line transfers energy to the rod and the drag of the reel will allow line to pull off as the pull from the fish increases. These factors allow an angler to use relatively light line for heavy fish. With all lines, the thickness will increase as the test strength increases.