A presentation on the finite-size effects (FSE) in collective dynamics in near-critical fluids and phase separating fluids. In computer simulations, FSE play an important role, especially close to a second order phase transition. Here, a comparative picture is provided on FSE in two different phenomena: collective transport (e.g., concentration current, bulk viscosity) and phase separation in a variety of systems (solid-solid, vapor-liquid, liquid-liquid transitions). An interesting universal nature of FSE is highlighted in this work.