Abstract: This invention provides a device and method for creating hydrodynamic cavitation in fluids which includes a flow-through chamber intermediate an inlet opening and an outlet opening; the flow-through chamber having an upstream opening portion communicating with the inlet opening and a downstream opening portion communicating with the outlet opening; the cross-sectional area of the downstream opening portion being greater than the cross-sectional area of the upstream opening portion; and a cavitation generator located within the flow-through chamber for generating a hydrodynamic cavitation field downstream from the generator. This invention also provides for a device for creating hydrodynamic cavitation in fluids wherein the walls of the flow-through chamber are removable mounted within the device and are interchangeable and replaceable with replacement walls having various shapes and configurations, thereby enabling the flow-through chamber to assume various shapes and configurations to affect cavitation.