This essay draws on the dialectical philosophy of science developed by ecologist Richard Levins and evolutionary geneticist Richard C. Lewontin—perhaps best known outside their fields of specialization for the essays collected in The Dialectical Biologist (1986)—to provide methodological orientation for research on planetary urbanization. Levins and Lewontin’s approach to dialectics emphasizes reciprocal co-determination and co-evolution: parts exist only through their relations to wholes; wholes are continually reshaped by the activity of their parts; and every process must be analyzed historically and across scales rather than reduced to a single, privileged unit of analysis or level of explanation. Building upon these methodological principles, we theorize planetary urbanization as an emergent, uneven, multidimensional, and contradictory process through which cities and the non-city environments that sustain them recursively transform one another and co-evolve, generating wide-ranging and often destructive biospheric reverberations. This anti-essentialist conceptualization requires the analytical decomposition and relational investigation of five key terrains of research on planetary urbanization: cities and agglomeration economies; hinterlands and operational landscapes; the human/non-human divide; periodization and geohistorical development; and the category of the planetary itself. This mode of analysis delineates a rigorously dialectical, non-reductionist research program for investigating the environments of planetary urbanization, elucidating how socio-ecological crises are generated through the very processes that produce and sustain urban life. It also opens a horizon through which to imagine, plan, and build alternative societal metabolisms—and the far-reaching political-economic, institutional, infrastructural, and sociospatial transformations they would require.