Abstract: The disclosed inventions relate to systems and methods for separating fluids into constituent fluid components. For example, apheresis is a process by which blood is drawn from a patient, the blood is separated and/or modified, and at least a portion of the blood is returned to the patient. In some embodiments, fluid separation can be accomplished in a continuous, in-line flow. For example, the fluid can separate in a direction transverse to the general direction of flow through the system. Baffles or spiral-shaped separation chambers can be used in a rotating fluid separation device.