A NOTE ON ENDOMORPHISMS OF MODULES OVER REDUCTIVE LIE GROUPS AND ALGEBRAS Andreas \v{C}ap Key words: Reductive Lie group, reductive Lie algebra, reducible representation. 1991 Math. Subject Classification: 17B10, 22E46. Abstract: We show how to apply the basic ideas of linear algebra to endomorphisms of modules over reductive Lie groups and algebras, thus obtaining notions of determinant, trace and more generally characteristic coefficients of such endomorphisms which are adapted to the situation. Finally, we show how these ideas can be applied to study locally trivial bundles of such modules over smooth manifolds.