Traditional observations of the properties of stars generally provide tests of only the gross aspects of stellar structure and evolution. The limitation lies in the amount and precision of the available data of relevance to the structure of the stellar interior, i.e., the determination of stellar effective temperatures, surface composition, luminosities and, in a few cases, masses. Additional constraints on the observed stars, such as the common age and composition normally assumed for stars in clusters or multiple systems, clearly increase the information. However, detailed information on the physics and processes of stellar interiors requires more extensive data, with a dependence on stellar structure sufficiently simple to allow unambiguous interpretation. Such data are offered by observations of stellar pulsation frequencies: they can be observed with great accuracy and their dependence of stellar structure is generally well understood. In particular, the richness and precision of the observed frequencies of solar oscillation are now offering a detailed view on the interior properties of a star.