This paper elaborates the view that cities as complex systems are typified by co-evolutionary behaviour and organization. As a consequence, cities change, adapt and maintain rich, diverse and varied strategies, sub-optimal behaviours, imperfect information, mistaken inferences and creativity. These aspects of cities are exemplified by reference to a set of simulation models developed by Allen and co-workers since the 1970s in a variety of domains (land-use and transportation), scales (a single city's dynamics and regional urban dynamics) and locations (Europe, the US, West Bengal and Nepal).