In this chapter, we argue that policy accumulation impinges on the process of policy implementation in distinct ways. First, the addition of new policy elements into existing policy portfolios can lead to a higher number of implementation tasks (expansion). Second, policy makers may close the gaps within established policy portfolios to become more encompassing by addressing a larger target group (densification). Finally, policy accumulation may require additional rules that specify how the numerous elements of a policy portfolio relate to each other. To deal with the resulting increase in implementation burdens under substantial resource constraints, implementers must develop adequate coping strategies, which result in implementation backlogs and delays as well as in patterns of selective implementation. Consequently, policy accumulation enhances the risk of increasing implementation deficits. Regardless of variations across countries, this threat is substantial and we will remain blind to this threat without adopting a new epidemiological perspective in implementation research.