This chapter examines the trajectory of just transition as the major contribution of labour unions to the practice and study of environmental justice. The concept of just transition adds a great deal of value because it includes work and workers, challenges the ‘jobs vs. environment’ myth, recognizes that green transitions have to be just, and explicitly fuses social and ecological justice. On the basis of a number of analytical concepts, I provide a historical genealogy of just transition that highlights its variability and ambition. As a result, this chapter will ensure that the reader will become familiar with the notion of just transition as a dynamic concept, understand its historical foundations in political practice and appreciate why, today, it is a contested concept invested with a variety of meanings.