Realism is an international relations theory that focuses on power and security. It has several core assumptions including that states are the main actors, their primary concern is survival in an anarchic system, and they will use whatever means necessary to ensure their security. There are different strands of realism including classical realism which emphasizes human nature and neo-realism which focuses more on the structure of the international system. Realism is criticized for being too state-centric and for justifying amoral actions in the name of survival. Overall, realism provides a lens for understanding international politics based on power dynamics between self-interested states.