The paper examines how the concept of 'excessive costs' has been interpreted in the implementation of industrial pollution control in the UK. Since 1984, industrial air pollution regulation in the EU has been guided by the framework concept of Best Available Technology Not Entailing Excessive Costs (BATNEEC). With the introduction of the Integrated Pollution Prevention and Control (IPPC) Directive in 1996, this has been replaced by the concept of Best Available Techniques (BAT). Despite the absence of the NEEC qualification, IPPC BAT includes excessive costs in its definition of 'available'. Both concepts require interpretation and both devolve potentially controversial decisions to the level of the individual site regulator. A central issue in interpreting 'excessive costs' is the relative importance of environmental cost-benefit analysis versus the ability of a sector to 'afford' environmental improvements. Also important is how such concepts can be operationalized by regulators who lack resources and depend upon industry for information. The paper provides a historical account of how these issues have been dealt with in the UK and argues that the key difficulties are far from being resolved. The paper concludes by assessing the implications for the future implementation of IPPC. Copyright