Bacteria swim through fluids by rapidly turning their flagella, and individual tissue cells migrate across surfaces in a cyclic process of expansion, adhesion, and retraction. These canonical types of motion, however, are not characteristic of cells within large, dense aggregates, such as bacterial colonies or the tissues of complex organisms. In this talk I will discuss tools and concepts of condensed matter physics that we have adapted to study the collectively generated forces that control multi-cellular motion within enormous cell aggregates. I will present research on bacterial biofilms, showing how they can spread by generating molecular gradients throughout the colony. I will also discuss collective motion within two-dimensional confluent sheets of mammalian tissue cells, showing how sub-cellular motions as well as multi-cellular forces, transmitted across long distances, each influence collective migration in different ways.