The unique ability of Pro or Pro-rich repeats to affect the stability and function of proteins has recently been highlighted by biophysical studies on fragments from prions, signalling domains and muscle proteins. Pro-rich regions have been observed to either occupy disordered states or adopt various helical structures; some are also able to undergo an environmental-dependent transformation between these states. Such a transformation could explain some of the inherent functional properties of the parent proteins and, additionally, can be efficiently exploited to generate novel temperature- and pH-switches in more conventional globular proteins.