The earliest legislation aimed to give comprehensive organization to the Italian education system was the Casati law, from the name of the then Minister for Education Gabrio Casati who drafted it. Promulgated by King Vittorio Emanuele II on 13 November 1859, the new law was designed to reorganize the school system in Piedmont and Lombardy, and was gradually and with difficulty extended to the other Italian regions. All legislation regarding education in Italy had been based on this law until 1923, when Giovanni Gentile, a prominent figure among Italian Idealist philosophers, introduced the reform that brought important changes to the school system, while maintaining various key features. To give a comprehensive view of the history of the teaching of mathematics in secondary schools in Italy over this 60-year period, we must look beyond the legislative, political and social factors, however important these undoubtedly are, to examine other factors not comprehensively studied as yet. On the one hand, we need to consider the role of mathematicians involved in advanced research; on the other hand, the role played by many different factors: secondary school teachers and their associations; the textbooks; journals concerning the teaching of mathematics; publishers' initiatives; conferences on teaching methods and practices; debates on methodology; international influences; teacher training. The purpose of this article is to give a general overview of the period, while describing in detail certain decisive moments, in order to show clearly the effects some decisions had, the debates they gave rise to, as well as the work carried out and the methodological approaches adopted by the mathematicians involved.