It is increasingly clear that we need to develop alternative development trajectories as current trends are unsustainable. This invited lecture at the University of Pretoria presented current trajectories related to natural systems such as carbon emissions as well as to social and economic systems such as inequality showing how these were unsustainable. It then sketched out key issues that would have to change in order for these trajectories to be directed towards more sustainable paths.  It sketched out key issues that would have to change in order for these trajectories to be directed towards more sustainable paths. The crucial role that cities, neighbourhoods and buildings play in this redirection was analysed and discussed. Finally, examples of built environment projects and tools were presented to illustrate how alternative development trajectories could be achieved.