All research begins with ontology. The agent/structure debate represents one of the most profound discrepancies and the least conclusive contests in this regard between Structural Realism and Constructivism since the late 1980s. Though politics is the terrain of competing ontology; definite and clear-cut answers are hardly attainable, a serious discussion on ontological issues can lead us to penetrate the assumptions deeply embedded in both theories. This article first examines theoretical significances in the agent/structure debate and next unpacks related concepts-such as agent, agency, and structure-as well as their origins in sociology. In the process, some conceptual controversies over state agency and ontological core of epistemological via media in Wendt's theorization are also briefly discussed. This article then turns to constructivist's two-front criticism against Waltz's Structural Realism in the agent/structure debate: Waltz is in fact an individualist and his ignorance about agency. After summarized and examined Waltz's arguments in respect of state as "unit", "emergence" of structure, "property" of structure, structural "effects," and structural "changes," this article concludes: though Waltz's theory, in strict terms of structural determinism, does not match with those thick Structuralism proposed by Levi-Strauss or Wallerstein, it is nevertheless better to see Waltz's theory as a thin Structuralism if Waltz's serious efforts to differentiate structure from agent and his theoretical responsiveness to Durkheim are properly appreciated. Waltz later makes many methodological amendments that lead his arguments more unit-level oriented; however, such efforts cannot bridge the existing gap between his Structural Realism and Constructivism since his ontological commitments about agent's agency and its potential to transform the structure are both wanting. Last, the significance and inevitability of theoretical parsimony implied in the illustration of level of analysis are highlighted; so constructivists, who cherish the complexity in their theory, may find unwarranted not to engage such an illustration. Thus, this article presents an illustration for the purpose of demonstrating ontological differences between Waltz and Wendt in the agent/structure debate.