Chitin and its deacetylated derivative chitosan are natural polymers composed of randomly distributed beta-(1-4)-linked D-glucosamine (deacetylated unit) and N-acetyl-D-glucosamine (acetylated unit). Chitin is insoluble in aqueous media while chitosan is soluble in acidic conditions due to the free protonable amino groups present in the D-glucosamine units. They both are highly valuable, and used in different fields of food, biomedicine and agriculture and so on. The success of chitin and chitosan in each of these specific applications is directly related to deep research into their physicochemical properties. Thereby this book is aimed to highlight the relationship between the physicochemical properties of the polymers and their behaviour. A functional characterization of chitin and chitosan regarding some biological properties and some specific applications are focused here in this book also.