The 21st-century city - defined by the duality of mass migrations to cities and continued sprawl - provides innumerable challenges and opportunities for architects, designers and planners today. Rapid environmental changes require scientific monitoring as forests and farmlands depopulate further; vast informal, self-organised urban settlements develop in the absence of master planning; and hyper-nodes monitor and influence everything through networked communications, media images, foreign aid and military might. Remote sensing and hand-held devices combine to create just-in-time delivery of design and planning services. These have the potential to shape and manage, as never before, vast interconnected ecosystems at local, regional and global scales. Close collaborations with scientists, decision makers and communities incite architects to realise new communication and networking skills. As the architect's role is transformed into that of a designer of the form of information, flows and processes rather than master planner, they will become the critical actor shaping the cities of this millennium. Presenting specially commissioned features on Dubai, Cochin, New York, London, Washington, DC and Barcelona, this issue of AD platforms emerging voices in architecture, science and planning. It also presents penetrating treatments of important aspects of the topic by specialists, such as geophysicist Christopher Small and US Forest Service social ecologist Erika Svendsen, and contributions by established urban designers and architects.