Cellulose, one of the most bountiful biopolymers worldwide since it is found in plant cell walls, is considered an environmentally friendlyEnvironmentally friendly and low-cost materialLow-cost material compared to other natural, semi-syntheticSemi-synthetic biopolymers, orSynthetic biopolymers synthetic biopolymersNatural biopolymers. This biopolymer has been extensively studied in biomedical applicationsBiomedical applications such as tissue engineeringTissue Engineering (TE) (3D printing)3D printing and wound dressing manufacturing, as well as in the pharmaceutical industryPharmaceutical industry for drug delivery due to its excellent physicochemical properties, as well as its safety, biocompatibilityBiocompatibility, biodegradabilityBiodegradability, biomimeticBiomimetic, and low toxicityLow toxicity. However, polyelectrolytePolyelectrolyte complexes, which are macromolecular structures obtained from the joining of two or more positively or negatively charged polyelectrolytes, have emerged as a strategy to solve the limitations presented by cellulosic materials and their derivatives by themselves. In this sense, one of the main advantages of using cellulose polyelectrolytePolyelectrolyte complexesCellulose Polyelectrolyte Complexes (CPC) lies in the fact that the possibility of toxicity is reduced by using chemical cross-linking agents, as well as improvements in their physicochemical properties. Therefore, this book chapter describes the fundamentals and applications of different cellulose polyelectrolytePolyelectrolyte complexes in the biomedical and pharmaceutical fields, mainly.