Since visible light activated composites first appeared in 1978 they have become internationally popular for the aesthetic restoration of anterior teeth. With an ever increasing number of composite formulations and curing lights available and as yet no widely recognized standard for measuring depth of cure, it is difficult to interpret claims being made by manufacturers. A laboratory technique using incremental measurement of surface hardness has been shown to provide a convenient means of assessing cure throughout the depth of a composite sample, which relates to the clinical situation. Using this method it is possible to demonstrate that physical and chemical formulation factors; the nature of the light source; and the control exercised by the practitioner, are all important in determining the quality of cure achieved and hence likely long term performance of the restoration. Light cured composites are now being more widely used in the restoration of occlusal cavities in posterior teeth, where the technique offers advantages of control, handleability and a lack of porosity unattainable in a chemically cured system. In large Class II cavities depths can exceed 7 mm, and in an application where performance is critical it is essential that a satisfactory level of cure is achieved. To obtain this, even with exposure times significantly longer than those currently being recommended, it is advisable to employ a layering technique for any cavity greater than 3 or 4 mm in depth.