The ancient province of Bithynia in north-western Anatolia centered on the fertile plain bordered by the Sea of Marmara and the Bosphorus in the west and the Black Sea in the north and, inland, stretched as far as the mountain passes east of Bolu and, to the south, down to the Uludag mountains in Bursa south of the Gulf of Izmit and the Köroglu Daglari hills. Geologically speaking, parts of its north-western edge still belong to Thrace, while to the east and south chalk and Paleozoic slate and limestone come together to form hills and mountains, peaking in the Uludag, the classical Bithnyian Olympus (2500 meters / 8205 feet). Verdant forests of beech, pine, fir, oak and rhododendron grow on Bithynia's well-watered Black Sea slopes and farming benefits from the sunshine higher up. More intensive farming in some parts is carried out by such settlers such as the Muhacir who were driven out of the Balkans, Caucasus, Crimea, etc. when the Ottoman Empire was stripped of these provinces in the late 19th century. One of Turkey's most densely settled areas, the Bithynian plain nowadays carries the main routes from Istanbul to Ankara and also forms part of the industrial sector of the north-west, much of it concentrated around Kocaeli, Adapazari and Bursa.

At the crossroads of Europe and Asia Minor, the territory of ancient Bithynia was constantly being fought over. Settled originally by the Thracians, around 550 BC. it was taken by the Lydians then later by the Persians. Since the wooded mountains of the north remained outside the dominion of Alexander the Great and his successors, Bithynia under the Seleucids was able to develop more or less independently and by the 2nd century BC, had become a kingdom in its own right, flourishing around its ancient capitals of Nikomedeia (modern Izmit) and Nicaea (modern Iznik). In 74 BC. it was made a Roman province. The Greek colony founded on the Bosphorus around 675 BC, where the Istanbul suburb of Kadikoy stands today, became the Romans' capital and in the Byzantine era, when it was the seat of the archbishopric, provided the venue for the fourth ecumenical council in 451. In the 11th century Bithynia was ruled by the Seljuks until the 14th century and then it has been an Ottoman territory with their capital in Bursa.