In this chapter, the authors trace the career of the justice theme in climate activism and the emergence of what they call a climate justice movement. Their overall aim is to elucidate what it has meant that "climate justice" became increasingly adopted by large parts of the climate movement after Copenhagen. While the Warsaw walkout made it clear that "climate justice" was no longer a dividing, but rather a unifying, concept in the context of global climate activism, it quickly became clear to the authors that radically different meanings were ascribed to it. In this chapter, the authors investigated how the idea of "climate justice" emerged in a movement context in grassroots environmental struggles in the United States in the 1980s, became embraced by transnational climate activists in the 2000s, and entered into the institutionalized milieu of the UNFCCC to become included in the Paris document in 2015.