Under the title "Complexity and Modernity" some of the most characteristic essays of Niklas Luhmann are presented, who, without fear of exaggeration, can be described as the most original German sociologist since Max Weber. This originality has much to do with the interdisciplinary openness of his thought, which, in this sense, not only follows but deepens by a fertile path, open to sociology by one of the main teachers of Luhmann: Talcott Parsons. The result of this is a radically innovative sociological theory, suggestive and, very often, provocative for conventional intellectual uses in the field of social sciences and humanities, to the extent that the point of departure and destination of it already it is not the idea of ​​unity, but, on the contrary, that of difference. Without pretending to be exhaustive, but trying to present a sample as significant as possible of the rich and complex sociological theory of Niklas Luhmann, professors Josetxo Beriain and José María García Blanco have selected and translated twelve essays, grouped around three of the major axes thematic of the work of the great German sociologist: one relating to «Communication and society»; another second referring to «Social differentiation and modern society»; and, finally, a third dedicated to «Sociology and man».