Prehistory and Nicomedia Period:
Kocaeli, erected on the intersection of crucial road and rail routes between Asia and Europe, is remarked to be one of the most important industrial settlements of
Marmara Region and our country. The historical background of Kocaeli begins in the early times of history. The town, which was first erected as Bithynia, was then named as Olbia, Astakos, Nicomedia, Iznikmid, Izmid and Kocaeli, respectively, throughout the historical process. The Megaraians, who moved from Thrace, created Astakos, settling in the locality of Bafliskele, located on the south of Izmit Gulf, in 721 B.C. The people of Astakos settled in 262 B.C. in the area, where is, in the present time, known as Izmit. This town, which served the Kingdom of Bithynia as the capital city until the collapse of the kingdom, was named as Nicomedia, after the founding father of the dynasty.

Roman-Byzantium Period
Dicoletian, the Emperor, declared Nicomedia as the capital of Eastern Rome in 284 A.D., when Nicomedia emerged as the fourth greatest metropolitan towns in the world after Rome, Antioch and Alexandria, with its armory, mint, dockyard, baths and temples. However, upon the declaration of Constantinople as the capital city of the empire by Emperor Constantine the Great and the read block of the accessing paths between Khalkedon (Kadıköy) and Izmit and preference of İznik (Nicea) for transportation by Emperor Justinian with military concerns, Nicomedia lost its significance.

Seljuks Period
Kocaeli was firstly captured by the Turks during the Seljuks' era slightly before the ending of the 11th century (1078). The final and definite establishment of Turkish rule over the city, which temporarily remained under the occupation of Alexious Comnenos during the Crusades, was during the reign of Orhan.

Ottoman Period

Kocaeli was made a part of the Ottoman territory by Akçakoca, a margrave of Orhan, the son of Osman, in 1337. Nicomedia was named firstly Iznikmid, then Izmid (Izmit) upon its going under the Ottoman rule. The city saw its most brilliant days during the reign of Kanuni Sultan Süleyman (Suleyman the Magnicifient) . The socio - economic life of Kocaeli blossomed as the railway line, which operated between Istanbul and Izmit in the 19th century and also included Haydarpafla (Istanbul) Ankara line from 1873, was passed through the city. The city became an autonomous sanjak (autonomous provincial territory in Ottomans) in 1888 and its name was changed as Izmit. In the aftermath, however, the area was named as KOCAELI (meaning, the domain of Koca) after Akçakoca.

A map showing Izmit in 19th Century.

Republic Period

Kocaeli, which lost its significance for a certain period of time due to the destruction and problems emerging as a consequence of the Great War, and which was occupied by the British (July 6, 1920) and the Greeks (April 28, 1921), was saved from the occupation by Turkish Army on June 28, 1921. Kocaeli has twelve districts, which are Başiskele, Darıca, Dilovasi, Çayırova, Izmit, Derince, Gebze, Gölcük, Karamürsel, Kandıra and Körfez. Kocaeli has, upon the declaration of Republic, become one of the most rapid developing cities, industrially, which is mainly thanks to its proximity to Istanbul. Izmit Paper Factory, opened in 1934 as the first paper production plant in Turkey, was followed by the second cellulose and paper factory in 1934, and the SEKA facilities were further enlarged in 1954, 1957 and 1959. Hence, Kocaeli has become an advanced industrialized zone of Turkey by the rapid industrialization until the present time. Another important hint about Kocaeli is that the town is the national time - setting reference for Turkey since the meridian 300 passes through the highway junction bridge in Köseköy.