In order to study weak continuity of quadratic forms on spaces of L² solutions of systems of partial differential equations, we define defect measures on the space of positions and frequencies.A systematic use of these measures leads in particular to a compensated compactness theorem, generalizing MURAT"TARTAR's compensated compactness to variable coefficients and GOLSE"LIONS"PERTHAME"SENTIS's averaging lemma. We also obtain results on homogenization for differential operators of order I with oscillating coefficients.