This dissertation presents a human geographic and socio-legal analysis of environmental legal mobilization in contemporary Poland. It examines how administrative law is being used by individuals and societal groups to enforce environmental protection commitments, and what the broader implications of interactions with the administrative legal system are for legal consciousness formation and social movement mobilization in a democratic society. I argue, first, that administrative law is an important site of inquiry for social scientists. Interaction with administrative legal processes shapes legal consciousness in consequential ways, because citizens are exposed to the state's power structure, prevailing governance logics, and the ways these might be reconfigured. I conceptualize administrative legal processes in environmental cases as contact zones between state, society, and the physical earth. Because Poland's administrative legal system is rather open to environmental advocates, and bolstered both symbolically and formally by the European Union (EU) governance system, meaningful opportunities arise for differently situated actors, including those not trained in law, to engage with the legal system and interact with others in the process of mobilizing their procedural rights. Environmental legal practitioners' contact with administrative legal processes shapes an evolving, spatially specific understanding of the form and substance of state power. In the dynamic interaction between direct observations of the physical earth, procedural rights' mobilization, and legal argumentation by various actors, spatial and temporal aspects of law are brought to the fore. Consequently, environmental legal practitioners, individually and collectively, build a critique of legal accountability mechanisms, the normative commitments of the state, and the prevailing political-economic system, expanding a performative critique of law already resonant in post-socialist societies. Environmental legal practitioners are actively shaping, and mobilizing others around, the indeterminacy and political possibility afforded by administrative legal processes. These dynamics are captured in the phrase "becoming a side," which refers simultaneously to Polish legal requirements for admittance as a party (strona, literally "side") to administrative proceedings, as well as to iterative processes of legal consciousness formation through practitioners' interactions with administrative law. More broadly, "becoming a side," refers to evolving relationships between state and society in which societal actors struggle to be involved in policy decision-making. In the latter part of the dissertation, I elaborate this more expansive sense of "becoming a side" by examining how members of grassroots movements move from local protest to proactive environmental and energy policy formation at the national level. I analyze two cases: mobilization against air pollution in Kraków, and the struggle against shale gas exploration in Żurawlów. Through the concept "becoming a side," I argue that using administrative legal tools and discourses, within formal institutional channels and beyond, is a way through which legal practitioners enact not only a re-framing of environmental advocacy, but also the very exercise of political liberal rights. They do so by sustaining a spatialized, substantive critique of state power, equipping others with skills and support to do the same, and thus enabling members of society to push for more governance accountability.