Policy makers worldwide have chosen to introduce many different instruments to mitigate emissions, including regulations, taxes and voluntary agreements. Documenting and understanding their choices is of enormous importance for academics and governors. This chapter defines policy implementation instruments as the various approaches available to governors to realize their policy objectives. It reveals that their choices typically depend on a multitude of factors, including their preferences, resources and prevailing institutional settings. It concludes by using policy theory to explore some of the factors that plausibly explain the choice of key policy instruments in the European Union, a self-styled climate policy leader, before discussing future theoretical, empirical and normative research needs in this hugely important and fast-evolving area of legal and policy analysis.