These proceedings reflect the main activities of the Paris Seminaire d'Algebre 1989-1990, with a series of papers in Invariant Theory, Representation Theory and Combinatorics. It contains original works from J. Dixmier, F. Dumas, D. Krob, P. Pragacz and B.J. Schmid, as well as a new presentation of Derived Categories by J.E. Bjork and as introduction to the deformation theory of Lie equations by J.F. Pommaret. J. Dixmier: Sur les invariants du groupe symetrique dans certaines representations II.- B.J. Schmid: Finite groups and invariant theory.- J.E. Bjork: Derived categories.- P. Pragacz:...