During embryonic development, groups of cells reorganize into functional tissues with complex form and structure. Tissue reorganization can be rapid and dramatic, often occurring through striking embryo-scale flows that are mediated by the coordinated actions of hundreds or thousands of cells. In Drosophila, cell rearrangements in the embryonic epithelium rapidly narrow and elongate the tissue, producing a tissue flow that doubles the length of the body axis in just 30 minutes. These types of tissue movements can be driven by internal forces generated by the cells themselves or by external forces. While much is known about the molecules involved in these cell and tissue movements, it is not yet clear how these molecules work together to coordinate cell behaviors, give rise to emergent tissue mechanics, and generate coherent flows at the embryo scale. To gain mechanistic insight into this problem, my lab combines genetic and biophysical approaches with emerging optogenetic technologies for manipulating molecular and mechanical activities inside cells with high precision. I will discuss some of our recent findings on how cellular properties and mechanical forces are regulated in the Drosophila embryo to allow (or prevent) rapid cell rearrangements and tissue flows during specific events in embryonic development.