The factors controlling heat flow into and from the head of an exhaust valve are discussed for the purpose of indicating the means by which temperature, the overriding important factor in exhaust valve failure, can be reduced. This analysis suggests the lines along which the designer and the metallurgist must co-operate. The causes of exhaust valve failure are reviewed, and the conclusion is reached that the majority of these are the result of corrosion fatigue. Even for resistance to burning the two properties inherent in resistance to corrosion fatigue, namely resistance to corrosion and good hot strength, are essential. Laboratory tests for determining resistance to high temperature corrosion fatigue and hot hardness are described, and some results given for exhaust valve materials. Corrosion fatigue resistance is considered to be so significant a factor in exhaust valve failure that it is suggested that laboratory tests to determine this property should form the basis for the development of improved exhaust valve alloys.