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Psychology’s most vital challenge is to strengthen its theoretical fundament. Popper’s program of competitive testing and the development of unified theories are 2 common routes toward this end. This article proposes a third and complementary route: the integration of already existing theories. To provide a systematic framework, I propose a 2-step theory integration program. The first step is integration of phenomena, that is, the study of how apparently disparate robust observations are theoretically connected, not just empirically correlated. The second step is integration of theoretical concepts, that is, the study of how apparently different explanatory concepts are linked. Links between phenomena or concepts include identity, nesting, and functional equivalence. Functional equivalence means that 2 or more psychologically distinct theoretical concepts can be shown to imply the same behavioral pattern by vicarious functioning. The 2-step program requires formalization and close attention to operational and conceptual definitions. It should not be seen as an algorithm that can be automatically applied, but as a heuristic method requiring creativity for building a network between theories.