This paper offers a theory of genealogy, explaining its rise in the nineteenth century, its epistemic commitments, its nature as critique, and its place in the work of Nietzsche and Foucault. The crux of the theory is recognition of genealogy as an expression of a radical historicism, rejecting both appeals to transcendental truths and principles of unity or progress in history, and embracing nominalism, contingency, and contestability. In this view, genealogies are committed to the truth of radical historicism and, perhaps more provisionally, the truth of their own empirical content. Similarly, genealogies operate as denaturalizing critiques of ideas and practices that hide the contingency of human life behind formal ahistorical or developmental perspectives.