Unlike the three Archaic and Classical gymnasia in Athens (Academy, Lyceum and Kynosarges), which were suburban, two gymnasia were built in the very heart of the city during the 3nd century BCE: the Ptolemaion and the Diogeneion. These complexes had a notable urbanistic significance, as far as they introduced some ‘modernity’ into the chaotic and old-fashioned centre of the town. Even if their exact location is still debated, it is certain that they intended to establish a firm connection, both topographical and ideological, with the oldest agora in the deme Kydathenaion, where the Prytaneion and the shrine of the hero-founder Theseus were located. An inscription of 122 BCE already attests to the high reputation of the Ptolemaion as an educational centre, such as the Lyceum and the Academy. The Ptolemaion and Diogeneion continued to be the headquarters of the ephebic training throughout the imperial age. Based on most recent and ongoing research, this paper will be concerned with the location, architecture and functions of the two gymnasia, providing also a topographic summary of the materials that originally belonged to the gymnasia and were reused in later structures. Focus will be on the epigraphic evidence, the main source of information for the architectural layout (compare e.g. SEG 21.397: stoa of the Ptolemaion; IG II2 1029 etc., library; IG II2 1011 and IG II2 2037: peribolos of the Diogeneion and a palaistra).