This document outlines three major approaches to studying international relations: traditional approaches of diplomatic history and philosophy, behavioralism, and alternative critical approaches. It discusses the contributions of several philosophers to international relations theories, including Plato, Aristotle, Aquinas, Hobbes, Rousseau, Kant, and how they addressed concepts like the state of nature, social contracts, and ideas for world order. Behavioralism proposes that individuals act in patterned ways that can be empirically tested. Critical approaches like postmodernism and feminism believe there are multiple realities and deconstruct major IR concepts.