The Social Accounting Matrix (SAM) is presented as a tool for measuring and modelling the activity of countries and regions. An SAM-based approach to the study of such activity enables it to be described both empirically and theoretically through numerical and algebraic versions of that same matrix. This approach is presented using the words of those responsible for its foundation: Richard Stone and Graham Pyatt. Applications of the SAM to Portugal and the Azores serve to underline the importance of ensuring consistency with the national and regional accounts when measuring and modelling the activity of countries and regions with SAMs. These applications are based, on the one hand, on SAMs for the Azores and Portugal, which have the same structure and identical characteristics and were constructed to serve as a database for a computable general equilibrium (CGE) model and, on the other hand, on a SAM for Portugal, the author’s area of research, which was constructed using the above-mentioned SAM-based approach. It is also underlined that such a task can be performed better if all of the part that is measured by the national and regional accounts is included in the study. These same accounts should be considered as the basic sources of information. The comparison of such applications underlines the importance of working with the institutional sectors together with the sectors of activity, as well as with the part relating to the secondary distribution of income together with the primary distribution of income. Also identified within this context are the risks of failing to consider important interactions in the activity of countries and regions, as well as the biases to be found in the analysis of the results.