Chitosan is composed of β-(1 → 4)-N-acetyl-D-glucosamine and β-(1 → 4)-D-glucosamine units. It is a partially deacetylated form of chitin, the second most abundant polysaccharide on Earth. This chapter includes a deep analysis of the most commonly used processes for its obtention from the shells of crustaceans, which are the most abundant natural source of chitin. Chitosan’s renewable origin, natural abundance, biocompatibility, and chemical versatility, together with the distinctive properties conferred by its polycationic structure, make it an attractive material for a wide range of applications. Considering that key properties of this biopolymer, such as its feasibility for chemical modification and processability, are strongly influenced by its molecular weight (MW) and degree of deacetylation (DD), their accurate determination is essential. In this context, this chapter provides a comprehensive overview of the analytical techniques most frequently employed for the determination of these structural parameters. Furthermore, the influence of MW and DD on key physical properties of this polysaccharide, such as solubility and viscosity, is discussed in detail. The chapter also summarizes the most frequently reported applications of chitosan and its derivatives in fields including food packaging, environmental remediation, and biomedicine.