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 as to the effectiveness of those strategies that had been introduced. This paper describes a methodology for assessing a country's progress in implementing a national sustainable development strategy for (NSDS) and for identifying potential areas for improvement. Five key principles of sustainable development and strategic planning are identified, and a set of assessment criteria are proposed for each principle. The results of applying the methodology in two Eastern European countries, Belarus and Slovakia, are reported. These case studies suggest that the proposed NSDS assessment methodology has considerable potential for strengthening sustainability planning at the national level. The effectiveness of the NSDS assessment methodology in strengthening national processes for sustainable development and strategic planning will also require greater transparency and accountability in governance practices. This suggests that progress in improving the quality of NSDS processes is likely to be conditional on broader considerations of institutional building and governance reform.