The externalisation dynamic widely attributed to these four systems is analysed here. External advisers are an important consideration in how policy advisory systems (PAS) operate and figure in analysis of their evolving nature. The external category is expansive, but this chapter is limited to analysis of two major external advisory units – think tanks and private sector consultants – with some attention paid to lobbyists and international bodies. There is consideration of the changing nature of the external environment as an important source of pressure on governments to open up or integrate external advice. The focus is on the displacement of public service advisers by outsiders and the shifts in influence to external units. Comparative analysis reveals important differences in the prominence of externals agents and underscores the elasticity of PAS and the Westminster tradition. Some countries and governments favour think tanks or consultants; others are more insular. Externalisation can arise because ministers are dissatisfied with internal advice or agendas driven by ideological preferences for smaller, more efficient government. Externalisation also functions as a pressure on government, which derives from international agencies, agreements, or contexts (e.g., Brexit) or from domestic proponents of ‘open’ government and policy.