The clinician involved in cataract assessment is often faced with a deterioration in visual performance which may or may not be totally attributable to the cataract. Neural changes may occur concurrently and contribute toward the reduced performance. This paper reviews the methods which attempt to discriminate between changes due to cataract development and changes produced by neural degeneration. New techniques have been developed with more emphasis on quantitative rather than qualitative assessment. These techniques are discussed critically with particular reference to their ability to distinguish between the contributions of optical and neural change. This has particular application in predicting the visual outcome after cataract surgery.