Indonesia consists of more than 17,500 islands, making it the largest archipelago in the world. About 6,000 of these islands are inhabited. Although Indonesia is surrounded by water, it still has land borders with Malaysia on the island of Borneo and Sebatik, Papua New Guinea on the island of New Guinea, and East Timor on the island of Timor. The five largest islands are Java, Sumatra, Kalimantan (the Indonesian part of Borneo), West Papua (part of New Guinea) and Sulawesi. The best known is probably the smaller island of Bali.

Indonesia is the world's 16th largest country by area, but with a total population of around 230 million it is the world's fourth populous country. Jakarta is the capital and largest city of Indonesia, it has a population of around 8.5 million, the metropolitan area, Jobodetabek, is the second largest in the world. Jakarta is located on Java, which is the main island of Indonesia, and where around 60% of the population lives.

Indonesia has the second highest level of biodiversity (after Brazil), and its fauna and flora is a mixture of Asian and Australasian species. Large species such as the tiger, rhinoceros, orangutan, elephant and leopard used to be abundant as far east as Bali, but numbers have fallen drastically and many are now critically endangered. Forest cover approximately 60% of the country, but in some places like on Java, the forests have largely been removed for human habitation and agriculture.

Indonesia also has a high level of sea and coastal ecosystems, including beaches, sand dunes, mangroves, coral reefs, sea grass beds, coastal mudflats and small island ecosystems.