This one-year study involved designing and implementing a problem-posing program for fifth-grade children. A framework developed for the study encompassed three main components: (a) children's recognition and utilisation of problem structures, (b) their perceptions of, and preferences for, different problem types, and (c) their development of diverse mathematical thinking. One of the aims of the study was to investigate the extent to which children's number sense and novel problem-solving skills govern their problem-posing abilities in routine and nonroutine situations. To this end, children who displayed different patterns of achievement in these two domains were selected to participate in the 10-week activity program. Problem-posing interviews with each child were conducted prior to, and after the program, with the progress of individual children tracked during the course of the program. Overall, the children who participated in the program appeared to show substantial developments in each of the program components, in contrast to those who did not participate.