Historically, national borders have evolved in ways that serve the interests of central states in security and the regulation of trade. This volume explores Canada–US border and security policies that have evolved from successive trade agreements since the 1950s, punctuated by new and emerging challenges to security in the twenty-first century. The sectoral and geographical diversity of cross-border interdependence of what remains the world’s largest bilateral trade relationship makes the Canada–US border a living laboratory for studying the interaction of trade, security, and other border policies that challenge traditional centralized approaches to national security. The book’s findings show that border governance straddles multiple regional, sectoral, and security scales in ways rarely documented in such detail. These developments have precipitated an Open Border Paradox: extensive, regionally varied flows of trade and people have resulted in a series of nested but interdependent security regimes that function on different scales and vary across economic and policy sectors. These realities have given rise to regional and sectoral specialization in related security regimes.