Chitin and its deacetylated derivative, chitosan, are natural polymers that have been proposed to be used in a
wide range of applications. Due to their natural origin, both chitin and chitosan cannot be defined as a unique chemical
structure but as a family of polymers which present a high variability in their chemical and physical properties. This variability
is related not only to the origin of the samples but also to their method of preparation. Due to this variability, the
success of the polymer on each application depends on the properties of the polymer. In a previous review, we highlighted
the relationship between the physicochemical properties of the polymers and their behavior. In the last three years, new
data regarding this topic has been published; this update includes this new information. Moreover, new applications (fiber
formation and cosmetic) that were not previously reviewed due to the extension of the topic are included.