Biopolymers are organic substances extracted from natural renewable sources, including plants, animals, and microorganisms. Chitin, the parent source of chitosan (CS), is the second most abundant natural biopolymer available in seafood-processing wastes and fungal cell walls. Pure and conjugated CS nanocomposites are extensively explored in environmental remediation, agriculture, food packaging, and biomedicine due to their affordability, superior heavy metals and dye chelation, biodegradability, flocculating, and film-forming and antimicrobial properties. CS reactive hydroxyl and amino groups enable surface functionalization for enhanced solubility over a wider range of pH, thermal stability, and mechanical strength. This chapter gives an overview on sources, extraction techniques, modifications, properties, and applications of CS.