Implementation research grew out of evaluation research. The Great Society policy reforms in the United States in the 1960s and 1970s stimulated a lot of evaluation research in order to estimate the effects of the new welfare state programs and to suggest improvements. Classic evaluation analyses raised the question whether it could be documented that a given policy intervention had any effect, and if so what effect. However, evaluation analysts often became frustrated that most studies actually showed little or no effect (Albæk, 1988). The classic interpretation of such findings was that the program did not work; it was based on a wrong causal theory. However, gradually the apparent failures stimulated another interpretation that maybe nothing was wrong with the causal theory behind the ...