The megacities around the world have been located in fertile river basins since the beginning of civilization. In recent years, population has been increasing drastically; as a result, people migrate to cities and live in vulnerable and marginal areas, thus giving rise to slums. Therefore, we would like to point out why and how megacities in Asia are vulnerable to flood. The vulnerability of the cities, especially the slums, is shaped by forces such as global changes, internal migration patterns, development practices and political instability, which constantly reshape the risk of floods, which may be frequent and disastrous. Hence, development choices with a strong scientific background, especially in the cities prone to monsoon, are necessary to protect development, and not only floods but all disaster risks should be regulated and mainstreamed into the development planning of cities.