This paper considers the principle that it is sufficient to establish causation in fact in tort to show that the defendant's act or omission made a material contribution to the harm. It examines the leading case, Bonnington Castings v Wardlaw, and other authorities and argues that the principle involves an application of the but-for test and not an exception to it. If exceptions to the but-for test are to be made, they should be clearly articulated and justified, as, for example, in Fairchild. A broad interpretation of ‘material contribution’ as establishing in some cases such an exception provides insufficient clarity and is certainly to be supported.