This chapter addresses the question of politicization and controversies in policy implementation processes. In contrast to the idea that politicization and controversies only occur in the policy formulation phase, the chapter highlights the importance of conflicts and debates around implementation. Politicization is defined as the process through which political processes become increasingly determined by partisanship, while controversies are public debates characterized by a strong salience and polarized opinions. Based on a literature overview of various policy analysis paradigms and illustrative examples, this chapter shows how politicization and controversies can dramatize the debate, thereby putting implementation activities under high pressure. It distinguishes various types of contentious implementation paths, depending on whether politicization happens at the political level (oversight), the delivery level (implementation), or the stakeholders' level (reception). It highlights future research avenues to better close the gap between policy formulation and policy implementation research.