Worldwide surveys indicate that in many countries there are high rates of failure and dropout in undergraduate mathematics courses. The secondary-tertiary transition in mathematics has been studied extensivey over the last decade, and yet, while there are many local initiatives that aim to support undergraduate mathematics students in their transition, there are very few success stories. Moreover, many university mathematics instructors argue that the gap between school and university mathematics is widening. Similar arguments are made regarding other transitions, for example from academics mathematics to workplace mathematics, or from classroom mathematics to everyday mathematics, and even from elementary school mathematics to secondary mathematics. How do different transitions in mathematics similar or different from one another? What makes transitions in mathematics relatively simple to some students and extremely challenging to others? One of the main challenges in understanding and resolving transition issues is that educators from the opposite sides of the transition typically do not interact with one another. How can we support different communities of practice in learning from and with one another about (teaching and learning) mathematics?