Kocaeli is mostly known by its district center, Izmit. The ancient name of the city is Nikomedia. Remains from Roman and Byzantine times may be seen here, among which are the citadel, the Temple of Augustus and the agora. There are lots of harbour facilities within Kocaeli, including the primary base of the Turkish Navy.

The city of Izmit is mainly an industrial center in Marmara region of Turkey with numerous industrial establishments surrounding the Gulf of Izmit. However, there are also many tourist sites in this bustling province. Kartepe is a winter sports resort that reaches a height of 1600 m, Kerpe and Kefken on the Black Sea coast have fine beaches, while Golcuk and Karamursel in the Gulf of Izmit are other beautiful sea resorts.
At Arpalik near the Eskihisar village in the Gulf of Izmit, there is a yacht berth place with a capacity of 60-80 yachts. Lake Sapanca in nearby Sakarya province is another natural beauty and a scenic place ideal for picnics. One of the towns of Kocaeli is Hereke, where the very best Turkish carpets are produced. The artistic carpets of Ottoman palaces were woven here only for the sultans. These carpets are priceless and well-known all over the world.

In 1999 August 17th, a terrible earthquake (7.4 Richter scale) hit the city at 03:02 local time while everybody was asleep and devastated the city and killed almost 10,000 people. It took the authorities many weeks before they could clean the rubble. Apart of the heavy loss of lives there was also a terrible loss in the economy as this city is the most industrialized area of Turkey. See the picture gallery for earthquake photographs.