Tokyo is built over the ancient Japanese city Edo (you can find Edo in cartoons or documentaries if you want to learn more). For now, the Tokyo, which you see today, is both Tokyo and Edo. In 2005, the population of Tokyo was 35.2 million while it reached 35.5 in 2015.
Right from the 18th Century, Tokyo topped the list of the world’s largest cities. Edo got its new name, Tokyo, in 1868. In fact, Tokyo has been destroyed a number of times by earthquake, wars and bombardments but again, the Japanese engineers are able to brand it.
Tokyo is probably going remain the world’s largest city in the days to come and so, it’s one of the cities of the future.