Fiber or fibre is a class of materials that are continuous filaments or are in discrete elongated pieces, similar to lengths of thread. Fibers are of great importance in the biology of both plants and animals, for holding tissues together. Human uses for fibers are diverse. They can be spun into filaments, thread, string or rope. They can be used as a component of composite materials. They can also be matted into sheets to make products such as paper or felt. Fibers are often used in the manufacture of other materials. Man-made fibers may come from natural raw materials or from synthetic chemicals.

Vegetable hi fibers are generally based on arrangements of cellulose, often with lignin: examples include cotton, linen, hemp jute, flax, ramie, and sisal. Plant fibers serve in the manufacture of paper and cloth.

Wood fiber, distinguished from vegetable fiber, is from tree sources. Forms include groundwood, thermomechanical pulp (TMP) and bleached or unbleached kraft or sulfite pulps. Kraft and sulfite, also called sulphite, refer to the type of pulping process used to remove the lignin bonding the original wood structure, thus freeing the fibers.

Animal fibers consist largely of particular proteins. Instances are spider silk, sinew, catgut and hair (including wool).

Mineral fibers comprise asbestos. Asbestos is the only naturally occurring long mineral fiber. Short, fiber-like minerals include wollastinite, attapulgite and halloysite.