With over ten million people, Seoul is South Korea's largest city and one of the largest cities in the world by population. Covering an area of only 605 square kilometres, smaller than London or New York City, it is one of the world's most densely populated major cities. Seoul is also one of the world's most digitally-connected cities with more Internet users than all of sub-Saharan Africa, excluding South Africa. The city is considered one of the top 20 "world-class cities".

The Greater Seoul metropolitan area, which includes the major port city of Incheon, has almost 23 million inhabitants making it the second most populous metropolitan area in the world, after Greater Tokyo. Almost one fourth of South Korea's entire population lives in the Seoul National Capital Area, and nearly one half in the Greater Seoul Area, making it the country's political, cultural, and economic center, as well as a center for international business. The city has played a key role in South Korea's economic development and has been referred to as the "Miracle on the Han River".

Seoul hosts more than three million registered vehicles and widespread traffic congestion is common. In recent years, the metropolitan government has undertaken extensive cleanup of the city's air and water pollution. The revival of Cheonggyecheon, a stream that flows through downtown Seoul, was a recent major urban beautification project.