Magmatic rocks of variable age and composition crop out extensively in Western and Northwestern Anatolia. In the present study we subdivide these granitoids according to their ages. The young granitoids (Late Cretaceous to Late Miocene) develop high-temperature metamorphic aureoles. Six isochronous belts are defined, which become progressively younger from north to south. The late Eocene to late Miocene granitoid belts are curved and open to the southwest. The old granitoids (Cambrian to Middle Jurassic) are present in the northwestern and northern parts of Anatolia. Many of their radiometric ages are disturbed as a result of later tectonic events responsible for the present-day structure of Western Turkey. Except For Cambrian granitoids, these rocks result from a series of northward-dipping subduction zones of Hercynian to Late Carboniferous age, along the Karakaya trench up to the Late Triassic, along and north of the Izmir-Ankara zone during the Middle Jurassic to the Late Cretaceous, and possibly north of the Hellenic subduction zone since the Paleogene.