We formulate an active-gel theory for multicellular migration in the extracellular
 matrix (ECM). The cells are modeled as an active, polar solvent, and the
 ECM as a viscoelastic solid. Our theory enables to analyze the dynamic reciprocity
 between the migrating cells and their environment in terms of distinct relative forces
 and alignment mechanisms. We analyze the linear stability of polar cells migrating
 homogeneously in the ECM. Our theory predicts that, as a consequence of cell-matrix
 alignment, contractile cells migrate homogeneously for small wave vectors, while
 sufficiently extensile cells migrate in domains. Homogeneous cell migration of both
 extensile and contractile cells may be unstable for larger wave vectors, due to active
 forces and the alignment of cells with their concentration gradient. These mechanisms
 are stabilized by cellular alignment to the migration flow and matrix stiffness. They
 are expected to be suppressed entirely for rigid matrices with elastic moduli of order
 10 kPa. Our theory should be useful in analyzing multicellular migration and ECM
 patterning at the mesoscopic scale.