Environmental permits come in many forms and control many activities. On occasion they can authorise one-off events, but more usually they provide authorisation for operations that will continue for some time. Even planning permission granted to construct a building has an enduring effect since it not only controls the building operation that may be completed within a short period but also determines what use of the land is authorised unless and until a new permission is sought. The precise life-span of many operations may be uncertain, but especially in view of the major capital cost of establishing chemical works, landfill sites, waste water treatment plants, etc., the operators' intention is to continue for a considerable time. Over that time circumstances may change so that the terms of the permit, or its very existence, are no longer considered appropriate. The aim of this paper is to explore the range of legal techniques that have been adopted for the variation of permits and the consequences of these in terms of enabling regulatory action and of protecting the various interests that will be affected by such exercises of power. The impact of such variation is most obviously felt by the operators, potentially threatening the viability, if not the fundamental legality, of their activities, but there is also an effect on third parties and the environment.