In this article we review the research on the flexible or adaptive use of solution strategies in school mathematics, with a focus on the most recent work in the field. After a short introduction, we provide an overview of the various ways in which strategy flexibility has been conceptualized and investigated in the research literature. Then we review the research that has looked at the relationship between strategy flexibility and task proficiency, followed by studies that analyzed the association of strategy flexibility and other learner variables, including learners' age, general mathematical ability, prior knowledge, executive functions, gender, and affect. Studies addressing the socio-cultural and educational embeddedness of strategy flexibility are reviewed next, and, finally, we discuss the intervention studies that have tried to stimulate learners' strategy flexibility by means of various instructional approaches. While this review reveals that strategy flexibility is increasingly recognized as an important and valuable construct in research and practice of mathematics education, and that recently substantial progress has been made in our understanding of this construct, there are many aspects of it that are still not well-understood and that need further investigation.