In this chapter three issues in the interpretation of the results of school effectiveness research are discussed: criterion choice, effect size and stability of effects. With respect to the first issue the overall conclusion is, that criterion choice and definition depend on the effectiveness perspective and the particular theory one wishes to corroborate. The issues of effect size and stability of school effects are treated both from the angle of a synthesis of available empirical results and from the angle of conceptual analysis. An overall evaluation of the available data on effect size and stability leads to the conclusion that school effectiveness models are not as shaky as certain critics would have it, but at the same time not established as firmly as enthusiastic school improvers treat them. Various suggestions as to the improvement of future school effectiveness research are offered, notably more refined research designs and more elaborate theory development.