This compendium contains a collection of key papers from the journal Energy Policy, offering a valuable reference point on the role of flexibility mechanisms in the mitigation of climate change. Originally published between December 1999 and August 2001, all of these articles concern particular aspects of the Kyoto mechanisms or variations on the theme of flexibility that have evolved elsewhere.For better or for worse, flexibility mechanisms have become a part of the institutional architecture of energy policy in the 21^st Century.As the contributions to this compendium illustrate, this fact has both positive and negative connotations. Flexibility mechanisms have the potential to unleash exciting market-led emission reduction initiatives, and to promote both technological and institutional innovation. At the same time, the complexity of establishing credible and robust arrangements, in which practicality and viability are balanced with appropriate safeguards against loopholes and gaming, should not be underestimated. In the longer term, the flexibility mechanisms may turn out to be unhealthy distractions from the much more important task of restructuring domestic energy systems. Or they might just turn out to be indispensable instruments for achieving the deep cuts in emissions that climate change demands. One thing, however, is abundantly clear: climate policy has guaranteed that energy policy will never be the same again.