Emergent collective descriptions of matter are central to our understanding of complex physical systems across a wide spectrum of energy scales. A prime example is hydrodynamics, encapsulating the behavior of a many-particle system into simple equations involving bulk properties such as pressure gradients and particle densities. From textbook knowledge, hydrodynamics is a macroscopic description for a system with a large number of microscopic constituents whose dynamics acts over scales completely separated from those characterizing the bulk motion. Our understanding of the applicability of hydrodynamics has been recently challenged by experimental observations made in mesoscopic systems of strongly-interacting particles. In the context of high-energy heavy-ion collisions, tell-tale signs of hydrodynamic behavior have been reported in so-called small systems, such as p-p and p-Pb collisions producing only a few dozen particles, whose hydrodynamic interpretation poses great conceptual challenges. At diametrically different temperature scales, experiments with few-body ultra-cold Fermi gases have demonstrated the onset of collective behavior and elliptic flow in systems with as few as 10 strongly-interacting atoms. How can hydrodynamic behavior be displayed in mesoscopic quantum gases that break all textbook requirements? The RRTF will engage in intense discussions aimed at improving our understanding of key theoretical and experimental results related to this fundamental problem. Subsequently, we will delineate cross-disciplinary research directions that should be pursued in future to shed light on the origin of these emergent collective phenomena and their universality.