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.