Traveling through the world's megacities has resembled the journey described in Invisible Cities by Italo Calvino. In this fictional masterpiece Marco Polo describes to Kubla Khan what he saw on his journey. After four decades of visits to megacities, it is also difficult to describe, let alone understand, the modern megacity, world city or global city. What they have in common are qualities of size, scale, shape and speed that are new in human development and unique in the history of globalization. For example, size proportionately affects both the physical and mental health of people and geometrically increases the global footprint of a city. We make cities but cities also remake us.

Megacities, determined by a population of ten million or more, vs. the global cities, determined by capital concentration, signal a new spatial partition of the world. The partition is represented within the city by formal vs. non- formal settlements or by vertical vs. horizontal growth.

These new (in)human settlements are full with both promise and peril, beauty and barbarism. More important than the fact that half the world's population is living in cities is the emergence of the very large city. It is in the megacities that the fate of global and planetary development rests. In such cities there is a dance between promise and peril, each partner uncertain of who is leading.

The promise of the megacity can be the diversity of experience, people and cultures; economies of scale; human capital; the concentration of science-technology-innovation, etc. The peril of the megacity includes both planetary and global threats, such as inequality, poverty, exclusion, pollution, violence or earthquakes. Martin Oppenheimer, father of the atomic bomb, recognized that "Today's city is the most vulnerable social structure ever conceived by man."

The bewildering methods of defining and measuring urban population suggest that even our conception of the city is indefinite. The megacity has been the subject of numerous horrific fictional accounts. But no one has managed to depict a city of twenty, thirty or forty million people as equitable and ecological. Such an idea cannot be sustained in the imagination.