Inter-level mechanistic explanations in the sciences have long been a focus of philosophical interest, but attention has recently turned to the compositional character of these explanations which work by explaining higher level entities, whether processes, individuals or properties, using the lower level entities they take to compose them. However, we still have no theoretical account of the constitution or parthood relations between individuals deployed in such explanations, nor any accounts of multiple constitution. My primary focus in this paper is to (...) outline a positive account of the constitution/part-whole relations between individuals posited in inter-level mechanistic explanations that takes constituents in the sciences to be ‘working parts’. Using this account, I then go on to illuminate the nature, and varieties, of multiple constitution that we find in the sciences and provide a starting theoretical framework for multiple constitution as well. (shrink)