How does government funding of corporate R&D affect the behaviour of firms?  Ongoing efforts to boost business investment in R&D demand better methods of evaluating the effectiveness of government policy instruments. Efforts to explicitly measure changes in the ways firms conduct R&D as a result of government policy – so-called “behavioural additionality” effects – have remained relatively underdeveloped. This publication explores the emerging concept of behavioural additionality and summarises results of a multinational effort to develop better ways of measuring it. Evaluations should investigate not only how much additional business R&D spending is stimulated by government support or how much additional output is generated as a result, but also how government funding influences the conduct and direction of business R&D.