Professional development is often determined by black and white thinking. Either issues are considered as being good or bad, or statements like teachers should or teachers must are transported. However, it is easily forgotten from which perspective the judgment is taken, surely it is not the teacher's one.Profoundly respecting and cherishing the teachers and their needs, allows for arriving at a vision of professional development that is for and with teachers, instead being simply about them. Such a vision encompasses support on many levels, as the following teacher statement so aptly points to:Edith: And if we then, and I am now talking of Mathematics Done Differently, are fortunate that such terrific mathematicians come into the school, this here is an experience that one could not have if one would drive elsewhere to attend an in-service course. [...] That we as teachers see that support from outside is provided, that we will be supported, is important [...]. This book presents the field of mathematics teacher professional development both from a theoretical and an empirical perspective. In particular, the initiative Mathematics Done Differently that has been run in Germany is presented, in whose context the data of the empirical study was gathered. The empirical findings led to postulating a model describing teachers' individual growth pathways and to providing implications for constructing practices that are based on what teachers really need.