Ideas are important in shaping the policy choices of governments. But many ideas that have not been successful in the past continue to be used by policymakers, and some good ideas tend not to be adopted. This Element will focus on why governments make these poor policy choices. We will discuss a number of examples of 'zombie ideas' that refuse to die, and then discuss the factors that are associated with their survival. Those factors occur at the elite, the organizational and the societal level. We will also examine some 'ghost' ideas that may well be successful but have a difficult time being adopted, and the factors that are associated with the exclusion of these ideas from the policy process.