Fiber is a concept that refers to or encompasses several carbohydrates and lignine that resist hydrolysis by human digestive enzymes and that are fermented by the microflora of the colon. From a practical point of view, fibers can be divided into soluble and insoluble. There is general acceptance of the concepts soluble fiber, fermentable, viscous and insoluble fiber, and non-viscous and barely fermentable fiber. The physiological effects and therefore the clinical applications of both fibers are different. In general, the insoluble fiber is barely fermentable and has a marked laxative and intestinal regulatory effect. Soluble fiber is fermented to a high degree, showing a powerful trophic effect at the colon level. Soluble fiber is also attributed a positive role in the carbohydrate and lipid metabolism due to the effects that this has at the intestinal and the systemic level on the glucose and the cholesterol metabolism. The goal of this article is to review the current concept of fiber based on the existing bibliography (it is thought that perhaps the current classification should be changed and that fiber should be talked about depending on its degree of polymerization), its physiologic effects and the possible indications that this may have from a clinical point of view, be this at the level of oral or enteral nutrition.