A net comprises threads or yarns knotted and twisted into a grid-like structure which blocks the passage of large items, while letting small items and fluids pass. It requires less material than something sheet-like, and provides a degree of transparency, as well as flexibility and lightness.

Nets have been constructed by human beings since at least the Mesolithic period for use in capturing or retaining things. Their open structure provide lightness and flexibility that allow them to be carried and manipulated with relative ease, making them valuable for methodical tasks such as hunting, fishing, sleeping, and carrying.

The oldest nets found are from the Mesolithic era, but nets may have existed in the Upper Paleolithic era. Nets are typically made of perishable materials and leave little archeological record. Some nets are preserved in ice or bogs, and there are also clay impressions of nets.

Originally, all nets were made by hand. Construction begins from a single point for round nets such as purse nets, net bags, or hair nets, but square nets are usually started from a headrope. A line is tied to the headrope at regular intervals, forming a series of loops. This can be done using slipped overhand knots or other knots, such as clove hitches. Subsequent rows are then worked using sheet bends, as shown in the diagram, or another knot. Some nets, such as hammocks, may be looped rather than knotted.

To avoid hauling a long length of loose twine through each knot, the twine is wound onto a netting shuttle or netting needle. This must be done correctly to prevent it twisting as it is used, but makes net production much faster. A gauge – often a smooth stick – is used to keep the loops the same size and the mesh even. The first and last rows are generally made using a half-size gauge, so that the edges of the net will be smooth. There are also knot-free nets.

Some nets are still shaped by their end users, although nets are now often knotted by machine.

When a hole is ripped in a net, there are fewer holes in it than before the net was ripped. However, the stress concentration at the edges of the hole often causes it to tear further, making timely repairs important. Mending nets by hand is still an important skill for those who work with them.

Nets may be made using almost any sort of fiber. Traditional net materials varied with what was locally available; early European fishing nets were often made of linen, for instance. Longer-lasting synthetics are now fairly universal. Nylon monofilament nets are transparent, and are therefore often used for fishing and trapping.