This chapter traces the evolution of thought on the causes of climate change from the early science on atmospheric physics to the adoption of the UNFCCC’s “common but differentiated responsibilities” (of nations) in 1992 to the responsibilities of non-state actors and a shared burden of mitigation and damages. The discussion then shifts to the responsibility of fossil fuel companies for their contributions to atmospheric carbon dioxide content and their early knowledge of their culpability for climate change, followed by corporate and trade association campaigns of deflection and disinformation in an effort to delay climate action. I discuss the emergence of litigation against Carbon Majors, which seeks redress for damages to property and injunctions based on human rights violations. Oil, natural gas, and coal companies face substantial legal (and moral) liability for the costs of climate change, even though other parties also share partial responsibility. Leading oil and gas companies understand that the climate crisis is existential and that, to retain their social license to operate, it is essential that they align their business models to meet the Paris Agreement’s 1.5°C target by mid-century.