Epithelial tissues must maintain organ integrity while continuously remodeling during morphogenesis, wound healing, and disease progression. At high cell density, tissues are often described as mechanically arrested, disordered, solid-like materials, and a large body of work interprets them within the framework of glassy and jamming physics. In this picture, cells are trapped by their neighbors (the “cage effect”), which should lead to subdiffusive dynamics and strongly slowed rearrangements, as in thermal glass-formers near the glass transition. Yet tissues do remain fluid over long timescales: cells exchange neighbors and eventually diffuse over distances much larger than their size, with transport that is often much closer to normal diffusion than what a passive glassy system would give. This tension between cohesion (solid-like at short times) and fluidity (liquid-like at long times) is the central question of this project. A plausible explanation is that active, non-equilibrium mechanisms intrinsic to cells (shape changes, self-propulsion, polarity dynamics, cell division and extrusion) continuously fluidize the tissue. However, these mechanisms are rarely compared on equal footing, and in particular the role of the coupling between cell geometry and cell motility remains poorly understood.