1. Introduction.- 2. Factors and Actors Behind Greater Complexity in Contemporary International Law.- 3. The Greater Complexity of International Law with the Intensification of Relations Among States and International Organizations.- 4. The Internationalization of Law from the Perspective of Infra- and Non-State Actors.- 5. New Features of the Internationalized Legal System: Expansion, Consolidation, Plurality, and Effectiveness.- 6. Challenges with Complexity: New Sources, Private Regimes and the Proliferation of Conflict Resolution Mechanisms.- 7. The problems of New and Old Concepts of International Law.- 8. Conclusions.