This article deals with theories in practice. A policy theory is defined as the total of causal and other assumptions underlying a policy. It can be reconstructed and represented in several ways, for instance by means of causal hypotheses, graphs, goal trees, and decision trees. A combination of these different ways of reconstruction is possible. The quality of a policy theory can be evaluated on the basis of several criteria, for example, the precision of formulation, the differentiation, the integration, the empirical value, and the legitimacy of the policy theory. In order to get more insight in the determinants of policy theories, it is important to compare them in longitudinal and cross-sectional research. The structure and quality of policy theories have effects on the contents, the process, and the results of a policy. It is a plausible hypothesis that the goal attainment of a policy will be higher as the precision, the differentiation, the integration, the empirical value, and the legitimacy of a policy theory are higher.