The introduction of a Medium Term Budgetary Framework (MTBF) has become a common element in Public Financial Management (PFM) reform. However, in many cases the results have fallen short of expectations as MTBF designs have been over-ambitious and have not taken adequate account of the starting conditions in terms of existing budget practice. This briefing note argues that MTBF reforms, which attempt to move too rapidly to a medium term budget based on the cost of present policy, are likely to encounter difficulties in many developing countries where existing policies and their costs are often poorly defined. The note suggests ways to make the approach simpler, and more achievable. By John Gray and David Hoole.