Koszul duality is a fundamental symmetry in mathematics that manifests itself in many fields in many different ways. This includes, for example, a relation between homotopy and (co)homology groups in algebraic topology (Eckmann-Hilton duality); a classification of vector bundles on projective spaces in algebraic geometry (the so-called BGG equivalence); a relation between the ring of differential operators and the de Rham algebra of differential forms on a smooth manifold in differential geometry; a relation between graded algebras and its Yoneda extensions in homological algebra (Priddy duality)... In this talk, I will say a few words about each of the above examples and try to formulate a general framework that unifies them all.