There is an emerging consensus that urbanisation is critically important to international development, but considerable confusion over what urbanisation actually is; whether it is accelerating or slowing; whether it should be encouraged or discouraged; and, more generally, what the responses should be. This working paper reviews some key conceptual issues and summarises urbanisation trends. It ends with a brief review of urbanisation and sustainable development, concluding that although urbanisation brings serious challenges, attempts to inhibit it through exclusionary policies are likely to be economically, socially and environmentally damaging. Moreover, with the right support urbanisation can become an important element of sustainable development.