At the Rio Earth Summit of 1992, governments undertook to develop and adopt national sustainable development strategies, as a key component of implementing the goals of Agenda 21. Only partial progress was reported at the 2002 World Summit in Johannesburg, with uncertainty in the effectiveness of those strategies which had been introduced. This paper describes a methodology for assessing the extent to which a country may be considered to have implemented an effective sustainable development strategy, with the prime aim of identifying weaknesses that need to be addressed. The methodology concentrates on evaluating those strategic planning processes that are operational in a country, taking into account any existing overall strategy document related to sustainable development, but not relying on it. It is intended to be used in three main ways: independent evaluation; evaluation by government as part of an improvement programme; and, ultimately, within an internationally agreed and verified mechanism for monitoring and accelerating the implementation of sustainable development strategies. To date, the methodology has been tested only in semi-independent evaluations, as described in a companion paper.