The Volvi basin (northern Greece) is a 5-km-wide graben of Neogene age. It has been chosen by European Union (Directorate General XII for Science, Research and Development in Climatology and Natural Hazards) as a test site (EURO-SEISTEST) for studying site effects in one of Europe's most seismic active areas. The site is located in the epicentral area of the 1978, Ms = 6.5, earthquake that struck the city of Thessaloniki. The geometry, velocity, and stratigraphy of the basin were not well known. Consequently, a number of geophysical and geotechnical studies were performed to determine the 2D geometry and the dynamic characteristics (Vp, Vs, Qs) of the valley. Investigations included 78 refraction tests, 20 surface-wave inversions, and 8 borehole seismic tests as well as geotechnical tests. This article presents the methodology used for the geophysical investigation and the results. In its center, the graben depth reaches 190 m, and at least four normal faults have been located along the section. The overall graben structure and the fault locations have been confirmed by the records of a shot generated at one edge of the valley and by the measurements of a local earthquake. Wave propagation appears to be strongly influenced by the thickness of the sediments, which in turn is controlled by fault displacements. Simple 2D modeling of the basin response was performed, and the results are in good agreement with weak-motion data, confirming the validity of the seismic structure inferred from seismic prospecting.