Since mathematical tasks are central to the teaching of mathematics, it is crucial to extend our knowledge of the characteristic features of tasks that are conducive to student development of problem-solving and reasoning abilities as well as conceptual understanding.   The aim of the dissertation is to investigate how different types of mathematical tasks affect student learning and choice of learning strategies. This is done through a twofold approach: 1) to test the hypothesis that tasks affording students the opportunity and responsibility for constructing knowledge are more effective learning tools than tasks for which the solution is presented, and 2) to analyse the educational message embedded in the teacher’s formulation of the mathematical tasks on the Internet.   The main conclusion is that the type of task students engage with is important for their learning of new things. The participants who were engaged in creating their own solutions were less successful during practice but performed better on the tests in comparison with the participants who were involved in solving the tasks with a given method. The results of the sub-studies indicate that in a learning situation consisting of repeated practice of a solution method, the results are closely related to the students’ cognitive ability. The investigation shows that tasks inviting the opportunity to be solved through creative reasoning, to a certain extent serve a compensatory function in relation to students’ cognitive resources. This means that the participants need not put in so much effort in the test situation if they have practiced creative reasoning.   One conclusion to be drawn from the study of the educational message in Internet documents, when it comes to teachers’ formulation of tasks, is that there are many teachers who design tasks that encourage young students’ creative reasoning. However, the educational message in the documents shows that the teachers demand relatively little of the students in the majority of the tasks. The result indicates that there is some uncertainty about how to formulate and use tasks to support the older student’s mathematical development. The way the tasks are formulated indicates a lack of discursive tools to clarify the intended educational situation. Thus, the qualities in the tasks are hidden resources. / Det övergripande syftet med avhandlingen är att undersöka hur olika typer av matematikuppgifter påverkar elevers möjligheter till lärande och val av lärandestrategi. Resultaten visar att i en lärandesituation som består av upprepad träning av en lösningsmetod är elevernas resultat starkt beroende av deras kognitiva förmåga. Uppgifter som ger eleverna möjligheter att lösa dem med kreativa resonemang verkar i viss mån kompensatoriskt i förhållande till elevers kognitiva resurser. Testdeltagare som tränat via kreativa resonemang har en lägre kognitiv belastning. Studien av lärarkonstruerade uppgifter på internet visar exempel på uppgifter som upplevs som användbara i undervisningen. Uppgiftskonstruktörerna tenderar att ställa förhållandevis låga krav på eleverna i merparten av uppgifterna. Resultatet tyder även på en osäkerhet inför hur uppgifter skall kommuniceras för att stötta elevers matematiska utveckling. De kvalitéer som finns i uppgifterna kommer därför i skymundan. Matematikuppgifter är centrala i matematikundervisningen. Det är därför viktigt att öka kunskapen om matematikuppgifter och hur dessa kan bidra till elevers möjligheter att utveckla sin matematiska kompetens. Väl utformade matematikuppgifter kan bidra till att minska elevers utantillärande.