Regarded as biofunctional poly- and oligomers having much higher potential than cellulose in many fields, chitosan and its derivatives have become of great interest thanks to the recent progress in chitin chemistry. Chitosan, which is a natural-based polymer obtained by alkaline deacetylation of chitin, is an interesting bioactive polysaccharide composed of many reactive groups. This gives huge possibilities of chemical modifications to create new derivatives with a broad range of physico-chemical and biological activities. This review gives insights into chitosan, chitosan derivatives and their biological activities. Recent progresses of their chemistry are also discussed as well as biodegradability and environmental assessment concepts applied to these derivatives.