Advances in environmental policy are closely intertwined with public participation and social movements. Individuals, organizations, industries, businesses, and other interested parties engage to exert influence in environmental decision-making. Their participation takes many forms in different systems and political arenas, ranging from comments on proposed regulations to lobbying to outside-system protests, with a wide range of effectiveness. This chapter reviews engagement in environmental policy, including what comprises good participation, decisions, and outcomes. Evolving forms of participation are considered amid contemporary challenges like climate change, focusing on activism and social movements, as well as the influence of polluting industries and interests aligned with them. A section focuses specifically on the environmental justice movement, which has served a critical role in achieving gains and shaping the broader environmental movement, although inequities in pollution-related harms persist. Across environmental issues, from the adoption of foundational environmental laws to contemporary issues at local, national, and international levels, it is apparent that an engaged public can wield significant influence. A fundamental lesson from environmental policy is that challenges to status quo systems, as are necessary to address issues like climate change, require substantial and sustained social movement participation.