This paper discusses so-called 'opportunity-use models of the effects of teaching'. More specifically, it compares different 'opportunity-use-models' with the aim to unveil existing conceptual fuzziness. The following issues will be critically discussed: a) the underlying conception of 'teaching', 'opportunity', and 'use', b) relations between 'opportunity' and 'use', c) the role of perception and interpretation, d) the localization of 'opportunity' and 'use' in a multilevel System, e) the role of the context, and f) the selection of teaching effectiveness criteria. In the conclusion an integrated opportunity-use model is developed.