In countries worldwide, the provision of policy advice to central governments has been transformed by the deinstitutionalisation of policymaking, which has engaged a diverse range of actors in the policy process. Scholarship should therefore address the impact of deinstitutionalisation in terms of the scope and scale of policy advisory systems, as well as in terms of the influence of policy advisors. This article addresses this gap, presenting a programme of research on policy advice in Whitehall. Building on Craft and Halligan’s conceptualisation of a ‘policy advisory system’, it argues that in an era of polycentric governance, policy advice is shaped by ‘interlocking actors’ beyond government bureaucracy, and that the pluralisation of advisory bodies marginalises the civil service. The implications of such alterations are considered against the backdrop of governance changes, particularly the hybridisation of institutions, which has made policymaking processes complex, prone to unpredictability and at risk of policy blunders.