In this chapter I present a study aiming to learn what kinds of mathe-matical reasoning are involved in the problems students have to face in the context of problem-solving instruction with the goal of better understanding both teachers’ practices and potential students’ learning. Indeed, problem solving is usually seen as a means to develop and assess students’ mathematical learning but can also be considered as an independent learning goal. In the context of this second purpose, the potential or at least the intended mathematical learning related to problem solving usually remained unclear. Nevertheless, some researchers highlight that problem solving can, among other things, help students develop mathematical reasoning skills. This leads me, in a first part, to define the mathematical reasoning and to charac-terize the different types of mathematical reasoning students can mobilized to solve mathematical problems at secondary level. Then, I focus the research on a course devoted exclusively to problem solving at secondary level, in the canton of Geneva (Switzerland). I analyze the problems of the official resources to identify the types of mathematical reasoning at stake. It helps to clarify the institutional purpose and to highlight and better understand the choices made by teachers. Then, I analyze the problems that teachers propose to students over a school year and how they articulate these problems. It leads to characterize teachers’ practices and to formulate some hypotheses about students’ potential learning related to problem solving. Finally, I give some leads to help teachers organize a teaching of problem solving linked to the mathematical reasoning at stake.