Amyloidoses are a very heterogeneous set of diseases, characterized by extracellular deposition of fibrillar proteins in different tissues. It is still a matter of debate whether the different forms of amyloidosis can share some common etiological mechanisms, or they are completely unrelated. The biochemical characterization of the protein component of the deposits provides a powerful system of classification for the different amyloidotic disorders and shades light on the molecular mechanisms of selective precipitation from soluble precursors and of tissue-specific deposition. Furthermore, identification and analysis of the genes coding for the precursors, and clarification of the kind of inheritance in some familial forms of amyloidosis, make prevention through genetic counselling and predictive diagnosis possible.