Part I. The Concept of Collective Security: 1. Collective security: a historical journey; 2. The morphology of collective security; 3. Triggers, actors and institutions; Part II. Collective Security Components: 4. States and collective security; 5. The United Nations; 6. Regional organisations; 7. Private military and security companies; Part III. Collective Security Tools: 8. The settlement of disputes and preventive security; 9. State-building; 10. Sanctions; 11. Military security; Part IV. Legal Management of Collective Security: 12. Law as internal facilitator, regulator or constraint; 13. Law as external facilitator, regulator or constraint; 14. The management of normative conflicts; Part V. Accountability in Collective Security: 15. Accountability in collective security; 16. International responsibility and liability; 17. Individual criminal responsibility; 18. Conclusion.