We installed 5 broadband and 45 short-period temporary seismic stations, distributed partly as a dense, 100-km-long, N-S linear array and partly as a regional network, throughout the Menderes Massif of western Turkey in order to study crust-upper mantle structure and seismicity. In this study, we have combined teleseismic waveform data from these stations with data from several permanent seismic stations to determine crustal thickness variations in the Aegean region. Receiver function studies at seven broadband stations, using the H-$kappa$ stacking method, have yielded crustal thicknesses and Vp/Vs ratios over a broad region of the Aegean. A more detailed crustal image was obtained in the central Menderes Massif, where we applied common conversion point stacking to receiver functions obtained from the N-S linear array. The results show a general trend of westward crustal thinning from 36 km in central Anatolia to 28--30 km in the central Menderes Massif to 25 km beneath the Aegean Sea. The results also indicate that crustal thinning in the Aegean is not uniform in the N-S extensional direction. The crust is thinner in the central Menderes Massif (28--30 km of crustal thicknesses) and the Cycladic Massif (25--26 km) than in surrounding regions where crustal thicknesses are 32--34 km. The long-lived elevated Moho under the metamorphic core complexes suggests that the lower crust in the Aegean region is at least 3 times more viscous than that in the Basin and Range Province, where the Moho is much flatter.