On the basis of studies reported elsewhere (SE 019 717 and 719) the author discusses the implications of research on cognitive development for teachers whose subjects involve an understanding of proportionality. Several common methods of teaching chemistry students to use proportionality in chemical problems are described. Research on the development of proportionality concepts is discussed, and basic results of Inhelder and Piaget, Karplus, the author, and others are briefly outlined. Laboratory tasks which can help students understand proportionality are described, and cooperation between teachers and psychologists is advocated. (SD)