We identify and analyse several dynamic implications of setting environmental standards such as to balance marginal costs and benefits. The adoption of such a regulatory approach is shown to effect (i) the speed of improvement of abatement technologies; (ii) the direction (in a sense to be defined) of that improvement; (iii) its source and the distribution of the rents from it; and (iv) the rate of development of defensive (averting) technologies. Existing views are thoroughly synthesised in the context of a simple diagrammatic model, several new results are derived and at least one conventional wisdom questioned. The message of the analysis for legislators and regulators is that cost–benefit balancing should be done with care.