The foal of a white horse is born black but changes to white as an adult. This is quite an amazing observation as – given its adult color – one would expect the foal to be white as well. However, nature has it that beings do not remain the same over time. Things develop over time and they change, be it foals or public policies. Public policy is a set of actions taken to address a societal problem that policymakers believe needs to be solved. Formally speaking, policy implementation is the part of the policy process that follows once a decision has been taken on how to solve a problem and the relevant authorities are called upon to put the agreed policy into practice. Of course, given the messiness of political reality, things are more complex than that. A lot can go wrong, or at least differently as planned. A political program can be met with the indifference of the politicians in charge and never be applied, it can encounter the opposition of implementing agents, produce unintended negative effects, be contested by the population, implementation can happen without formal political decisions… As the saying goes, a paranoiac is just a person who knows what they are talking about, which undoubtedly applies to political scientists, who are well-aware of the gap between a policy on paper and in practice. These uncertainties make policy implementation a worthy and rich field of study to understand collective human behavior and the construction of society. Public Policy Implementation is a core element of the policy process. It is a highly political phase in which policies already decided upon undergo fundamental changes due to the discretion that policy programs provide to the implementing actors. Policy success and effectiveness depend on the implementation process as much as it depends on the policy design that stands at the beginning of the implementation process. The purpose of this handbook therefore is to shift the focus away from the merely institutionalized arenas of policy-making, and to bring a wide collection of cutting-edge knowledge to shed light on the everyday chaos of policies on the ground.