The purpose of the paper is to present three different contributions to a general model theory which the authors think as relevant to systems research based on systems theory, cybernetics and constructivism. This also implies that the three contributions are shown to be of use both in the study of systems that operate by models, as well as in the practice of designing models.

The text is conceptual and mainly of a descriptive and referring character. Besides the sections on the three contributions to a model theory, the authors have inserted sections that relate them to systems theory. This is also achieved by the help of some simple models.

The paper also points to models as important instruments in constructing and selecting information to observing systems, and the authors hope that the text can be helpful in developing better knowledge of how models work.

The text is about observing systems that observe by models, and that is probably a somewhat new perspective. It is hoped that the paper can be inspiring to further studies on this matter.