While Pierre Duhem (1861–1916) is now mostly known as an influential general philosopher of science and an early historian of medieval science, he also made significant contributions to theoretical physics, and spent his entire scientific career working on a general thermodynamics or energetics that was meant to include (in a certain sense) also classical mechanics. His own conviction of the correctness of his energetics — in contrast to other theoretical options, esp. atomistic theories — seems to run counter to the prima facie instrumentalist flavour of his main philosophical work The Aim and Structure of Physical Theory. Recently, philosophers have come to appreciate realist tendencies in Duhem’s thought, although the extent of his scientific realism remains controversial. In this talk, I review Duhem’s general philosophy of science, paying particular attention to issues of realism and instrumentalism. I, then, turn to Duhem’s energetics, esp. to the problem of stability, to illustrate one way in which realism and instrumentalism are intertwined in Duhem’s practice of theoretical physics.