Polymorph control: past, present and future

The appearance and disappearance of polymorphs is no longer a mysterious and inexplicable process. Although methods for polymorph control are still imperfect, there is a large armoury of methods that can be used to tackle this important and challenging problem. We survey the methods and their successes over the last few years. A single substance can have multiple crystal forms: some useful, some awkward and even some dangerous. We outline recent approaches to the control of polymorphs.