The essay analyses the use of foreign law by Constitutional Courts. Part I addresses the causes of the use of foreign law primarily but not exclusively within the Western Legal Tradition. Part II is devoted to the classifications of legal systems based on the method and purposes of the use of foreign precedents. Part III develops a critical analysis of traditional classifications with particular reference to Markesinis’s tripartition explained and discussed in the previous Part. Finally, Part IV deals with the use of international law with a view of marking differences and similarities between the resort to foreign law and the use of international sources.