Relativistic Hydrodynamics is the description of fluid motion in regimes where relativistic effects are important. This is the case for fluids moving at high velocities or interacting with very strong gravitational fields, such as in the physics of black hole accretion disks or neutron star mergers but also in the microscopic dynamics of high-energy heavy-ion collisions. Although the first formulation of hydrodynamic equations dates back to the beginning stages of relativity theory, many mathematical problems remain wide open. In particular, the development of the theory of relativistic "viscous" fluids was slow and mathematical progress only made recently. The purpose of this Mini-Workshop was to bring together a diverse group of researchers, including specialists in nonlinear PDEs and physicists, to jump-start the mathematical development of this field. This allowed for a vital exchange of ideas between mathematics and physics communities.