The research on the nature of the fundamental constitutional principles and on their relationship with values and rules starts from a historical reconstruction of the concept of principle, even from different perspectives. The study of constitutional principles focuses on the analysis of the evolution of the modern understanding of the principles of law, in particular the general ones, identifying the differentiation with respect to the contiguous conceptual categories of value and rule. This historical reconstruction - focused, in particular, on the legality of the same principles - relates to the notion of law principles in the rationalist natural law, goes further through the notion of general principles of law in the positivist theoretical model, dominant until the first half of the twentieth century, and ends with the emergence of neo-constitutionalism and the recognition of the normativity of constitutional principles. The analysis is conducted on the basis of the main doctrinal positions emerged within the German, French, Spanish, Portuguese, Latin American and Italian doctrine. According to the position taken by the author, the axiological dimension of the international and European orders permeates and pervades the structure of the fundamental principles enshrined by national Constitutions; it is also stressed that the fundamental principles which can be found within the international and European block of constitutionality arise and work in a relationship of mutual complementarity and harmony with the fundamental principles of the domestic constitutional systems.