In this issue we begin an occasional series on public health at different levels.

For a long time it has been the convention to consider health and health care from the perspective of the District Health System derived from local government administrations and allied public sector organisations. In this thinking the district general hospital has loomed large and to an extent it has been complemented by moving down to primary care and up to the national level. Today's post-modern and global world requires new thinking about health and health care systems that recognises changing demographic and environmental contexts, societal change and a shift in emphasis from all encompassing structures to looser, more variegated networks. In such networks particular issues need to be addressed at the population and organisational level that most make sense. Subsidiarity and additionality are the key principles when looking at public health from a neighbourhood, community or village level; from that of borough, town or city, conurbation, region, nation state, global region or the world system itself. We begin with two British perspectives from the national level and from the conurbation of Greater London. We would be happy to consider other contributions from other levels and other parts of the world.