Despite much recent attention being given to strategic environmental assessment (SEA), there are considerable difficulties moving from a useful concept to widespread and enduring practice. This may be partly because the proponents of the policies, programmes and plans (PPPs) and the decision-makers remain unaware, or unconvinced, that SEA can add value to the existing processes in PPP development, assessment and decision-making. This paper attempts to respond to these difficulties by defining an overarching concept of SEA and a set of principles to assist in the evolution of effective methodologies. A broad range of SEA methodologies will be needed to adapt to the particular PPP-making context and these must be grafted onto the existing PPP process.