Emma Castelnuovo (1913-2014) was an Italian mathematics teacher of grades 6 to 8 in secondary school. During the crucial period of the 1950s and 1960s, when important reforms were proposed, she was involved in significant events such as the first CIEAEM meetings and the Royaumont Seminar. She was an active contributor to the development of international cooperation in mathematics education and was one of the first authors in the journal "Educational Studies in Mathematics." Thanks to pioneers such as Emma, the figure of teacher-researcher took shape and the milieu of mathematics education was forever changed. The main aim of Emma's work as an innovative teacher was to actively involve learners. In outlining Emma's action as a mathematics educator, we will pay particular attention to the use of concrete materials as this was a favorite means by which she pursued her aims.