cancers are known to invade collectively [@cheung2013collective], it remains an important open question whether this theory also describes the sensory behavior of tumor cell collectives.

Physical models of collective migration
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Metastatic invasion is a process of cell migration. Collective invasion, in turn, is a process of collective migration. Therefore, it is important to understand not only the collective sensing capabilities of tumor cells, but also the properties of their collective migration—and ideally the relation between the two. From a physical modeling perspective, describing collective cell dynamics is an interesting problem, because often rich and unexpected behavior can emerge from a few simple interaction rules between cells. Even in the absence of sensing, simple models have successfully explained observed collective behaviors such as cell streaming, cell sorting, cell sheet migration, wound healing, and cell aggregation [@kabla2012collective; @szabo2010collective; @basan2013alignment; @janulevicius2015short]. Here we focus on the collective dynamics that emerge when sensing plays a key role. In this case, a sensory cue results in polarization of a cell or cell collective via one of a variety of mechanisms [@jilkine2011comparison], and the dynamics are directed, i.e. migratory.

Mechanisms of collective migration
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Broadly speaking, the mechanisms of collective migration can be divided into 