The UK government has identified the land‐use planning system, and development plans in particular, as potentially powerful instruments for integrating national sustainability objectives into strategic decision making at local levels. One method for achieving this is through the use of so‐called ‘sustainability appraisals’, which are an extension of the established system of environmental appraisal used by planners since the early 1990s. A national framework is outlined in Planning Policy Guidance Note 12. Local authorities are now expected to conduct an environmental appraisal of their development plans which covers sustainable development issues. However, little research has been conducted on the effectiveness of current guidance in meeting this aim. By evaluating the implementation of sustainability appraisals nationally, this paper suggests that while government advice to appraise is generally being applied, the actual use of key sustainability principles in practice is rather variable. It then discusses these findings in relation to the changing context of appraisals in the UK and other national planning systems.