Functional appliances have been used for over 100 years in orthodontics to correct Class II malocclusion. During this time numerous different systems have been developed often accompanied by claims of modification and enhancement of growth. Recent clinical evidence has questioned whether they really have a lasting influence on facial growth, their skeletal effects appearing to be short term. However, despite these findings, the clinical effectiveness of these appliances is acknowledged and they can be very useful in the correction of sagittal arch discrepancies. This article will discuss the clinical use of functional appliances, the underlying evidence for their use and their limitations.