The Belgian psycho-pedagogue Ovide Decroly (1871-1932) acquired worldwide renown as one of the pioneers of the New Education Fellowship. In the period between the First and the Second World War, this organization was the forum par excellence for new education in Europe. This article analyzes Decroly's writings in order to examine the relationship between special and new education. In Decroly's view three factors determined education, either special or new: the child itself (the biological factor), the environment including human beings (the sociological factor), and the way in which both factors influenced the child's psychological development (the psychological factor). The child was first and foremost a living creature, with mental functions that had to develop in a social environment. This implied that one had to try to find the elements for a better understanding of the child within biology, sociology and psychology. Pedotechnics, in particular psychogenesis, was the way through which Decroly explored these domains of science. (Contains 71 footnotes.)