We live in a time of polycrisis, when governments all over Europe face multiple and serious security threats that differ in form. Military action involves the confrontation of armed forces; the global economy requires the movement of money and trade across borders; and climate is changing due to heat-inducing substances injected into the atmosphere. These threats affect many levels of society from the individual household worried about inflation to global policymakers grappling with climate change. Threats to national security can come from other states; non-state organisations; trans-national processes such as international markets; or natural phenomena. To understand how collective security threats differ, this chapter contrasts differences in their histories that remain significant today. It examines how security threats differ in their causes and in how they impact society. To protect against threats, prime ministers must rely on inputs from a variety of institutions when making decisions about security. Decisions are often the outcome of conflicts between the multiple institutions concerned with security.