'Operads' are mathematical devices which model many sorts of algebras (such as associative, commutative, Lie, Poisson, alternative, Leibniz, etc., including those defined up to homotopy, such as $A_{infty}$-algebras). Since the notion of an operad appeared in the seventies in algebraic topology, there has been a renaissance of this theory due to the discovery of relationships with graph cohomology, Koszul duality, representation theory, combinatorics, cyclic cohomology, moduli spaces, knot theory, and quantum field theory. This renaissance was recognized at a special session 'Moduli Spaces, Operads, and Representation Theory' of the AMS meeting in Hartford, CT (March 1995), and at a conference 'Operades et Algebre Homotopique' held at the Centre International de Rencontres Mathematiques at Luminy, France (May-June 1995). Both meetings drew a diverse group of researchers.The authors have arranged the contributions so as to emphasize certain themes around which the renaissance of operads took place: homotopy algebra, algebraic topology, polyhedra and combinatorics, and applications to physics.