Kocaeli is located in Marmara region and mostly known by its district center, Izmit. The ancient name of the city is Nikomedia which is dating back to 8th century BC. Remains from Roman and Byzantine times may be seen here, among which are the citadel, the Temple of Augustus and the agora. These are mostly in ruins and hard to see, but some findings from excavations in the area can be found at the archaeological museum.

The city of Izmit is mainly an industrial center in Turkey with numerous industrial establishments surrounding the Gulf of Izmit. However, there are also some tourist sites in this bustling province. Kartepe is a winter sports and skiing resort that reaches a height of 1600 meters, Kerpe and Kefken on the Black Sea coast have fine beaches, while Golcuk and Karamursel in the Gulf of Izmit are other sea resorts. There are several parks in the city, with amusement parks and outlets.

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.

A very interesting historical fact is that Hannibal, the Carthaginian general who defeated Roman army in Italy in the 2nd century BC, died in the old city of Libyssa, today's Gebze district. Today a commemorative grave is erected for this great general of ancient times.

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.