Cells are intrinsically stochastic creatures. Not only individual cells behave randomly during growth, migration and chemoresponse, but also they differ from each other even if derived from the same source. Despite this variability, multicellular organisms are capable of performing highly regulated, coordinated activities which are crucial to maintain the normal functionality of complex life systems. A crucial step to understand these two apparently conflicting facts is the emergent behavior coming out of cell-cell interactions. In this talk, I will present two salient examples of such collective behaviors observed with mammalian cells. In the first example, we studied the spatial-temporal dynamics of fibroblast cells collectively responding to ATP molecules. By using single cell calcium imaging in microfluidic devices, we characterized the correlations within the cell populations and identified the importance of pacemaker cells. We also found two channels of cell-cell signaling: gap junctions and diffusing molecules induced different collective responses. Unifying these observations yields a novel picture of chemoresponse on the population-level, and paves the way of understanding information process by a bio-network. In the second example, we studied in vitro models of cancer invasion. Two driving forces of cancer invasion were tested. First in a microfabricated landscape of micropillars, cell migrate to the top of the pillars because of the need for more spaces. On the other hand, due to cell-cell interactions, invasion can be suppressed. These two competing factors resulted in distinct invasion profiles for metastatic and non-metastatic prostate cancer cells and interesting social behaviors of the two cell species. In another model, we studied breast cancer cell invasion into extracellular matrix (ECM) driven by nutrition gradient, closely mimicking the physiological conditions. We found in this case cancer cells exhibit very similar group behaviors as for bike racers in Tour de France. This observation suggests an underlying optimization mechanism used by the cancer cells which may be explained through a game theoretical approach. _______________ Bo Sun is a post-doctoral associate in Department of Mechanical and Aerospace Engineering and the Department of Physics at Princeton University. He received his Ph.D. degree in 2010 from the Department of Physics at New York University, under the supervision of Prof. David Grier.