The chapter, which considers the reception of Malthus from the end of Napoleonic rule to World War I, is divided into two parts. The first consists of two sections dealing with the reception of Malthus’s political economy. Two facets of it were discussed by Italians: the underconsumptionist theory of crises and rent theory. Basically, Malthus’s perspective was dismissed in favour of Say’s. The second part, encompassing six sections, is devoted to the principle of population. In particular, the fourth section surveys the debate before 1848, the fifth section addresses the shift towards a ‘progressive’ Malthusianism occurring in the 1850s, and the sixth section traces the Catholic thinkers’ changing assessment. The methodological approach of Messedaglia and Pareto is discussed in the seventh section. The following section examines how Malthus’s law combined with Darwinism and socialism at the end of the century. Finally, the appeal to the law made by neo-Malthusians is depicted.