Patient and public involvement (PPI) is now firmly embedded in the policies of the Department of Health in England. This article commences with a review of the changing structures of PPI in English health and social care, largely in terms of their own explicit rationales, using that as a spring board for the development of a general theoretical framework. Arguing that all democratic states face major dilemmas in seeking to meet conflicting demands and expectations for involvement, we identify the diverse and sometimes conflicting cultural and political features embedded in current models of involvement in England, in a context of rapid delegitimation of the wider political system. We identify some of the major inherent weaknesses of a monolithic, single-track model of patient and public involvement in the management and running of health and social care systems. Although the mechanisms and methods for delivering this may vary we suggest the model remains fundamentally the same. We also suggest why the current structures are unlikely to provide an effective response either to the pluralism of values, ideologies and social groups engaged in the sector or to the valuing of lay knowledge which could potentially sustain the social networks essential for effective participation and service improvement. The article proposes a four dimensional framework for analysing the nature of PPI. These dimensions, it is argued, provide the co-ordinates along which new 'knowledge spaces' for PPI could be constructed. These knowledge spaces could facilitate and support the emergence of social networks of knowledgeable actors capable of engaging with professionals on equal terms and influencing service provision.