The first part of the study offers a critical review of the principal theories on the establishment phase of the Thersilion and Theatre at Megalopolis, and it revises the dating of both structures on the basis of stylistic analyses. These indicate that the Thersilion, consisting of a hypostyle hall and its »portico« of the columned prostasis, was built between 360 and c. 345 BC. By contrast, the Theatre, for whose koilon it was necessary to heap up very- considerable amounts of earth, and the skenothek were both added to the Thersilion no earlier than the last quarter or towards the end of the 4th century BC. Benson revealed in 1892 that the Thersilion was substantially restored in the Hellenistic period. Recent analysis of the column plinths in the interior of the hall has permitted more precise conclusions. All the surviving plinths date from the restoration of the building - which consequently must have been built entirely anew, albeit on the old ground-plan. Ignored in previous research, dowel and chisel holes on the plinths allow us to infer the size and appearance of the columns, remains of which have survived. As regards dating, correspondences in workmanship between the Hellenistic elements of the Thersilion and elements of the Theatre's proskenion suggest their contemporaneity. The paper seeks to demonstrate that the rebuilding of the Thersilion was made necessary by the destruction of the old building in 222 BC by Cleomenes III of Sparta and that it was undertaken as part of a fairly extensive building programme including the new theatre proskenion and commissioned by Philopoimen in the years after 190 BC. In this period Philopoimen seems to have commissioned the reconstruction of the Stoa of Aristodamos.