The chapter offers an overview of different approaches to the professional development of mathematics teachers. Starting point is the expert-novice-approach establishing both a distinction between experts and novices as well as an attempt to characterise expertise. The question of how to conceptualise professional compe- tence of mathematics teachers is subsequently deepened by a detailed description of related prominent theoretical and empirical approaches followed by a discussion of central empirical results. The chapter closes with a short summary of recent research emphases especially taking into account the various discussions on teachers’ pro- fessional development at ICME-13.