The paper describes a design research study with experienced mathematics teachers of post-16 low-attaining students. The study explored the use of generic collaborative mathematics tasks in promoting professional development. I describe the theoretical basis for the design of the tasks, the tasks themselves and a professional development programme in which they were used by teachers drawn from 44 colleges. Teachers that used many of the tasks reported profound changes to their practices and this was confirmed student reports and classroom observation. Teachers' beliefs about mathematics, teaching and learning both constrained the ways the tasks were implemented and were challenged by the affective and cognitive outcomes. The variation in these outcomes are examined and explained from the evidence.