This chapter assesses the cost of populist protectionism, beginning with the traditional measures of welfare cost from trade restrictions, as well as the institutional disruption it often entails, which amplifies these costs. One distinctive impact of Trump’s trade policies as well as from Brexit, for example, is the uncertainty it creates in the business environment, which itself tends to diminish business investment and trade. Populist protectionism, by flouting established rules, also tends to provoke retaliation, further compounding its cost. The systemic cost of eroding long-standing trade practices and norms also diminishes trust between populist governments and other countries, which may move global trade toward discriminatory, defensive trade blocs. Reduced trust may spill over into nontrade issues in which cooperation is required to solve cross-border or global problems, such as with the coronavirus crisis. Populist restrictions on immigration also have negative trade effects in terms of inefficient labor allocation, reduced output, and diminished trade opportunities.