The formation of living organisms results from mechanical processes that first affect its basic components, tissue cells. As opposed to passive materials, living cells actively respond to the mechanical perturbations occurring in their environment. Cell adhesion and migration require the ability of cells to generate active forces. I will first present physical concepts of single cell adhesion and the unexpected cellular responses to mechanical cues such as stiffness sensing. Besides the mode of single cell migration, detailed knowledge obtained over the past 30 years suggests that at least one additional mechanism is important for cell translocation within tissues: the movement of cell groups consisting of multiple cells connected by cell–cell junctions. I will then focus on collective cell behaviors which plays a pivotal role in biological systems in regulating various processes such as gastrulation, morphogenesis and tissue organization. By combining experimental approaches and numerical modeling, I will present how physical constraints modulate collective behaviors of epithelial cell sheets and how these approaches can be used to probe the mechanical properties of epithelial tissues.