Measurement estimation skills are of significant importance for everyday life. In the last decades a lot of research results were generated describing students’ estimation skills and strategies. Surprisingly, little attention has been paid to the basic question which types of situations are relevant for a valid conceptualization and operatio-nalization of measurement estimation skills. Some studies refer to the basic structure of estimation conditions described by Bright (1976) whereas others ignore this question, though it is central to ensure validity of the empirical data. Following validity criteria and based on existing empirical findings on estimation strategies, we developed a comprehensive model of measurement estimation situations. This model provides a basis for the development of valid tests on measurement estimation skills as well as for the development of learning environments.