The Trump administration, elected on a wave of populism, has moved at a rapid fire pace to disrupt the global order. Most controversial has been its repeated attempts to ban entry into the United States by travellers from six countries in the Middle East – all Muslim – for 90 days and to stop accepting any additional refugees. It has appealed directly to both economic nationalism and identity politics, traditionally the core elements of populism. The United States is not alone in facing an upsurge in populism. Older British voters outside the big cities voted to turn their backs on Europe and regain control of immigration. France and Italy both face surging populist movements. Populism is not a new phenomenon. It has happened repeatedly in history, but what it is and when it happens is still widely debated. Bart Bonikowski is an expert on populism who has studied it both in history and in the present across countries. He brings a broad comparative understanding of what populism is and what it means. Bonikowski is Professor in the Department of Sociology at Harvard University and Resident Faculty at the Minda de Gunzberg Centre of European Studies.