Summary: This paper describes insights on how to promote mathematical reasoning in problem solving based on the mathematical experiences of participants in a long-term study in which the students engaged in strands of well-defined, open-ended mathematical investigations, as a context for research on the development of particular concepts and ways of reasoning. Over the years, the students demonstrated ways of working in which sense making became a cultural norm and collective and individual sharing and justifying of ideas was a common practice. The paper examines the environment that enhanced the development of these and other qualities. The insights address aspects of task design and researcher role in the studentsâ€™ mathematical activity. Both were central in enhancing the studentsâ€™ engagement in thoughtful and meaningful problem-solving activity. The paper addresses the relationship between problem solving and mathematical reasoning from the perspective of the long-term study and provides an examination of studentsâ€™ problem solving over time, both from a behavioral and introspective perspective.