The main techniques developed to characterize chitosan are recalled. The interaction of chitosan with oppositely charged surfactants was investigated giving very important surface activity effects, A few chemical modifications are described and the new properties obtained are mentioned: alkylation gives amphiphilic polymers having interesting thickening behavior; grafting cyclodextrin (chit-CD) gives a polymer able to include hydrophobic molecules; grafting adamantane gives an amphiphilic polymer able to specifically interact with chit-CD forming a temporary network with gel-like behavior.