Conventional wisdom has it that top-down studies of mitigation costs are too pessimistic while bottom-up studies are too optimistic, In examining mitigation cost studies for Western Europe, this paper finds this interpretation to be flawed, The paper expands the analytical framework for interpreting mitigation cost studies, evaluates the treatment of no regrets options and other input assumptions in the various studies, and includes findings from recent integrated top-down/bottom-up modeling assessments not covered in the IPCC report, Based on these new elements, it is found that mitigation costs tend to be overestimated not only in top-down studies, but also in bottom-up studies, Given this pervasive bias toward pessimism, the body of existing mitigation cost assessments for Western Europe can be read as follows, Over the next three to four decades, a combination of tax shifts, market transformation instruments and policies to reform subsidies and internalize non-climatic externalities could cut carbon emissions in Western Europe by 40-50% relative to the base year, at no loss or at a gain in GDP, Averaged over the medium term and including a phase-in period, an economically optimal rate of implementing Western Europe＇s no regrets potentials is found to be in the neighborhood of 2% per year relative to base year levels.