To avoid dangerous anthropogenic interference with the climate system, drastic mitigation measures have become necessary. But who should do what and how much of it should they do to help the global effort to reduce global greenhouse gas emissions? This chapter addresses this question by specifying the identity of duty-bearers, the content of their mitigation duties, and how demanding these duties are. It identifies five families of agents and explains that each individual and collective agent has specific duties to contribute to mitigation measures: individual agents, nation-states, subnational jurisdictions, supranational formations, and economic corporations. For each family of agents, arguments for and against mitigation duties are scrutinized, with the objective of presenting a detailed account of burden-sharing climate justice. In addition to investigating the duties held by individual agents and nation-states, the two families of agents that have attracted most attention from climate justice scholars so far, this chapter also proposes to focus on three new agents in order to turn philosophical discussions on climate change in new directions: cities, the World Trade Organization, and carbon majors. The polycentric approach to climate change governance and the related normative framework of multiscalar justice seem particularly promising in terms of finding new ways to promote climate justice in a context of failure to bring climate change under political control at the national and international levels.