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The world is urbanizing at a breakneck pace. But not all regions are moving at the same speed. Most population growth today and tomorrow will occur in the sprawling cities and slums of Africa and Asia. Just three countries — China, India and Nigeria — will account for 40% of global growth over the next decade. Meanwhile, many cities in North America and Western Europe are actually shrinking.

The pace of this urban revolution is mesmerizing. Just 3% of the world’s population lived in cities in the early 1800s, compared to over 50% today. Yet future urbanization will take place not only in megacities, but in small and medium-sized cities of the Global South. There are tremendous opportunities in these fast-growing settings, but also unsettling risks. Some cities are especially susceptible to insecurity.

All cities are fragile. The intensity of their fragility, however, varies considerably across time and space. Some cities – Aleppo, Caracas, Kabul, or Mogadishu – are affected by acute fragility and are close to collapse. Others – Abuja, Baltimore, Dhaka, and San Salvador – are also at risk, albeit to a lesser degree. Even cities like Amsterdam, London, New York, Paris, and Tokyo are not immune.

Fragility occurs when city authorities are unable or unwilling to deliver basic services to citizens. It is triggered by a rupture of a city’s social contract. So what tips cities over? The intensity of fragility is conditioned by an accumulation of risks. And some risks – the pace of urbanization, income and social inequality, youth unemployment, homicidal and criminal violence, poor access to key services, and exposure to climate threats – are more serious than others.

Which cities are most fragile? This straight-forward question is surprisingly hard to answer. Part of the problem is that there is no agreed definition of what constitutes a city. More fundamentally, there is a shocking paucity of data on the vast majority of the world’s cities. While there are dozens of think tanks and private firms that claim to gather data on cities, their geographic coverage is surprisingly thin.

Today’s debate on cities is limited to the world’s 30-odd megacities and the 600 cities driving international economic growth. They’ve expanded with dizzying speed: these cities constitute a new layer of global governance. Yet there is virtually no discussion about what is happening in the other roughly 3,400 cities with over 100,000 residents. And there is a veritable silence about the other 50,000 cities and urban localities around the world. This is worrying because it is cities whose names you’ve never heard of that will shape the future.