We use teleseismic receiver functions to investigate the crustal structure at two locations in western Turkey using seismic data recorded on small arrays of temporary broad-band seismographs. The results from these analyses are compared with receiver function results from the GDSN station ANTO on the Anatolian Plateau in central Turkey. The crust is similar to 30 km thick in the region of western Turkey where active normal faulting reveals present-day extension in the upper crust and alkali-basaltic volcanism reveals recent extension within the subcrustal lithosphere, The crust is similar to 34 km thick further east where crustal extension is still evident but less pronounced. In the Anatolian Plateau, which is not currently extending, the crust is similar to 38 km thick, The level of extension estimated from these measurements of crustal thickness implies a p-factor of similar to 1.2. This value agrees with the amount of extension estimated in the upper crust from the integrated seismic strain rate (beta-factor of similar to 1.3), from surface faulting (beta-factor of similar to 1.25) and from the amount of extension in the subcrustal lithosphere estimated from the volcanism (beta-factor < 2), all indicating that the extension is approximately uniformly distributed vertically throughout the lithosphere, The Moho transition in this region appears to thin slightly as the degree of extension increases westwards.