Chitosan was proposed as a drug carrier for mucosal administration in ocular, buccal, nasal, gastroenteric and vaginal-uterine therapies based on its bioadhesive properties and biodegradability in vivo under the action of hydrolases. Examples are the delivery of acyclovir via ocular administration, and the delivery of 5-aminosalicylic acid to the colon. Microparticles may need to be cross-linked to retard their degradation in acidic media; yet cross-linking with glutaraldehyde introduces cytotoxic characteristics and depresses bioadhesion. Alternative cross-linking approaches are discussed along with the suitability of chitosan for the oral delivery of vaccines.