Ernst Mach’s Die Geschichte und die Wurzel des Satzes von der Erhaltung der Arbeit is now widely regarded as occupying a pivotal position in his oeuvre. Erik C. Banks called it a ‘roman à clef’. More generally, it is safe to say that the law of energy conservation played a central role in Mach’s thought. He frequently referred to it in his publications to illustrate how science works. This chapter has a two-fold aim. First, it sets his reflections on energy conservation against the background of its nineteenth-century history. Mach appears as a particularly astute observer of his own time. Second, it relates Mach’s comments on the law to his overall philosophy. Although in the second half of the nineteenth century the importance of the law of energy conservation was generally acknowledged, there was no consensus on what the law actually meant. Its name, discoverer, formulation, justification, and implications were all subject to debate. Mach was at the same time a participant in this debate (interpreting the law of energy conservation) and a commentator on it (interpreting debates on the law of energy conservation). In the process, he developed a new understanding of scientific meaning as grounded in communication, practice, and history.