Abstract French physiologist Claude Bernard (1813–78) was among the most distinguished and influential of the nineteenth-century French scientists. Although initially a poor student, Bernard was fortunate to learn the techniques of experimental physiology from his mentor, François Magendie, who was then the leading physiologist in France. Bernard soon made major discoveries concerning the role of the pancreas in digestion, glycogenic function of the liver in carbohydrate metabolism, sympathetic nervous system, mechanism of action of curare, and oculosympathetic paresis. He also developed a systems approach to biology and came to view animals as complex multicomponent and multilevel systems that were neither mystical nor reducible to straightforward physicochemical phenomena; this framework led to his concept that life depends not only on a milieu extérieur (external environment) in which the organism is situated but also perhaps more importantly on a milieu intérieur (internal environment) in which the tissue elements live, an antecedent to the later concept of homeostasis. Bernard's most important and enduring theoretical contribution to biology, however, was his masterpiece, Introduction à l'étude de la médecine expérimentale (Introduction to the Study of Experimental Medicine, 1865); this was intended as the preface of an intended magnum opus, Principes de médecine expérimentale (Principles of Experimental Medicine), which unfortunately was never completed. Bernard's scientific achievements were recognized by numerous honors and the first state funeral for a French scientist, all of which was achieved despite immense personal costs including a failed marriage and bitter conflicts with antivivisectionists.