polymer chains, respectively. The term \"kitosan\" (Kite-O-San) was firstly written by [PERSON] in 1894, to design deacetylated chitin [6]. Indeed, chitin is not soluble in water or other common organic solvents, but can be converted in CS after hot alkaline deacetylation in a solid state [2]. The degree of deacetylation (DD), which is the percentage of d-glucosamine units with respect to the total number of monomers (glucosamine and \(N\)-acetyl glucosamine), defines the frontier between chitin and CS. Conventionally, the DD value of CS is usually higher than 50%. The resulting CS, which is a polycationic polysaccharide, is soluble in dilute acidic media (2 < pH < 6), on the contrary to chitin [7]. In industrial processing, CS is mainly extracted from crab, shrimp shells, squid pens, and crustaceans by acidic treatment to eliminate calcium carbonates, followed by alkaline deproteinization [5]. The demineralized and deproteinized chitin is then submitted to a second alkaline treatment at high temperature before an optional decolorization step using hydrogen peroxide, sodium hypochlorite, or acetone [5]. All these acidic and alkali treatments are extremely hazardous for the environment and not sustainable. Enzymatic deacetylation is often considered as an ecofriendly alternative to alkaline deacetylation, but is not really industrially developed at the moment [6]. New commercial sources of CS from fungi (from Kitozyme company) and insects (from Ynsect company) have recently appeared on the market to valorize some by-products (mushroom wastes or cuticles of insects from new protein production chains). They are based on more green processes compared with those used by traditional CS production chains. The physico-chemical properties of CS depend on its molecular weight (from approximately 10 to 1000 kDa), DD (in the range of 50-95%), and sequence of the acetamido and amino groups. It has been used in a large range of applications due to its unique physicochemical properties, but also its low toxicity, biodegradability, biocompatibility, high adsorption capacity, and microbe resistance [4; 8; 9]. Indeed, the different functional groups of this polycationic polysaccharide can be modified with a wide diversity of ligands. Among them, the amino group (-NH\({}_{2}\)) functionality is available for numerous chemical reactions, including reactions with aldehydes and ketones (Schiff's base), chelation of metals, alkylation, sulfonation, carboxymethylation, grafting acetylation, quaternion, etc. [10; 11; 12]. The numerous hydroxyl groups (-OH) are also, as for all polysaccharides, available for chemical modifications, such as sulfonation, carboxymethylation, phosphorylation, or hydroxyethylation [10; 11; 12; 13; 14]. All chemical groups along the backbone can be cross-linked using specific agents to give \"chemical\" hydrogels. They can also interact with each other due to ionic and hydrophobic interactions, molecular entanglements, or hydrogel bonds to generate physical hydrogels [9]. Moreover, macromolecules of CS can produce self-assembled structures based on hydrogen-bond networks formation in aqueous solutions, leading to fibers. Conformational variations of these CS assemblies have been reported to depend on local environmental changes around CS (e.g., pH, temperature, types of salt, and types of acids). All these reactions offer to CS a great potential as biosourced materials, biomaterials drug/enzyme delivery vehicles, tissue engineering scaffolds, adhesives, texturing agents, support for enzyme immobilization, bioactive agents etc. This review focuses on the fundamental uses of all forms of CSs, i.e., polymer, oligomer, native, and chemically modified, in a large variety of applications. Thus, bioadhesive applications, antimicrobial activities, adsorption, and chelation in the wine industry, as well as developments in medical fields or biodegradability, have been detailed for highlighting the potential of chitosan and derivatives.