Because of his public leadership of the philosophe party in eighteenth-century France, Voltaire stands today as the iconic example of the French Enlightenment philosopher. Denis Diderot (1713–1784) is often seen as Voltaire’s second in that role since it was around both men that the Enlightenment philosophes rallied as a movement after 1750. The epochal project, which Diderot jointly pursued with Jean le Rond D’Alembert, to “change the common way of thinking” through a comprehensive Encyclopedia, or Reasoned Dictionary of the Arts, Sciences, and Trades provided the emergent philosophe movement with the cause around which they would coalesce. Diderot also fought vigorously with Voltaire on behalf of the Encyclopédie project and its principles, becoming as a result a public leader of the Enlightenment philosophical party in France alongside Voltaire. He also worked, like Voltaire, as a writer and critical intellectual who willingly positioned himself against the grain of established authority, and one who used philosophy as a vehicle for political and social activism. Yet Diderot’s philosophy pursued many more agendas and dimensions than Voltaire’s. He also left behind a corpus of philosophical writings that marks him out as arguably the most sophisticated of all the Enlightenment philosophes, and as one of the great philosophical thinkers of the eighteenth-century. Despite the obvious sophistication of Diderot’s philosophy, his legacy has suffered because of the historical differences separating his writings from the discipline of philosophy as it is practiced today. Enlightenment philosophie was something very different from what professional academic philosophers mean by that term today, and Diderot’s writings are often ignored by modern philosophers because they do not appear to be philosophy as they know it. Like many Enlightenment philosophes, Diderot also worked as an homme de lettres first and foremost, and only as a philosopher narrowly construed in certain instances. He also never authored any recognizable work of “systematic philosophy” if by that term we mean writing in the vein of his contemporaries such as David Hume in his Treatise or Immanuel Kant in his Critiques. Yet Diderot made important contributions to modern philosophy, and if they are to be grasped, the historical differences separating his writing from philosophy today must be transcended, and his eclectic manner of working accepted and embraced. Diderot wrote works that we recognize today as philosophy, but he also wrote a great deal more than that, and the challenge presented by his eighteenth-century philosophie is to see the modern philosophy contained in all of it. For Diderot did not simply write plays, art criticism, prose fictions, and highly imaginative works of literature alongside his work in philosophy; he pursued philosophie through these ostensibly literary works as well. He experimented with genres, including philosophical genres, when crafting his thought, and his writing overall is redolent with a self-consciousness that makes any easy separation of his explicitly philosophical writings from his literary work well-nigh impossible. His publishing habits were similarly complex, for as a writer who suffered personally under censorship that made the traffic in illicit ideas a prosecutable offense in Old Regime France, Diderot often had very good reason to leave his work unpublished—and very often did. At the same time, censorship alone does not explain the peculiar mix of published and unpublished writings found in Diderot’s oeuvre. This historical complexity has given rise to some difficulty in assessing Diderot’s writings according to the disciplinary canon of modern philosophy. Condillac, Helvétius, and d’Holbach are the Enlightenment philosophes most commonly studied within philosophy departments because their writings appear to conform better with conventional understandings of what philosophy should look like as a genre and a linguistic idiom. By contrast, the works of Diderot tend to be studied only in literature or history departments. This is unfortunate, for the treatment of Diderot’s philosophie as something different from modern philosophy has cut contemporary philosophers off from the work of one of the most sophisticated, subtle, and complex philosophical thinkers of the eighteenth century. To some extent, the way in which Diderot’s philosophical work employs different genres but also, challenges the idea of genre itself, has made it seem (perhaps too easily) congenial to a more “Continental” philosophical tradition, and foreign to a more formally oriented “analytic” tradition. But that would ignore Diderot’s naturalistic commitments and the role the Encyclopédie played, e.g., in the self-image of philosophers of science in the Vienna Circle. Our entry seeks to go beyond such oppositions in dealing with Diderot as a philosopher. Neither perspective alone fully grasps the richness of Diderot’s contributions to modern philosophy, so in order to fully situate his philosophie within philosophy writ large, a flexible and reflexive attitude regarding his writings must be adopted. Every text in Diderot’s oeuvre needs to be treated as a participant in both his philosophie and his philosophical work, and our conventional understanding of the boundaries isolating art and literature from science and philosophy also needs to be suspended because very often these modern distinctions do not apply in Diderot’s case. He also manifests an awareness of the new and emergent disciplinary taxonomy arising at the time, targeting his philosophie on many occasions at an interrogation of these developing epistemological divisions. This reflexivity often makes his thought even more relevant today than it was when it was written. To capture the complexity of Diderot’s philosophie as philosophy, this article adopts this reflexive approach. It will proceed in two parts. An overview of Diderot’s life and major texts is offered in Part I so as to present his work and writings as particular episodes in a coherent eighteenth-century life and career. To simplify the reading of this biography, the text is offered in a two-level presentation. A short overview of the highlights of Diderot’s life and work is offered in Section 1 to give readers a schematic overview, but a more extensive presentation of his biography is available in the Biographical Supplement. A comprehensive analysis of Diderot’s major philosophical preoccupations as revealed in his writings is then offered in Section 2 so as to outline the contours of Diderot’s place within Enlightenment philosophie and modern philosophy overall. This is followed by brief concluding remarks in Section 3.