An overflow chamber (2) is provided for use in a combined sewer overflow. An inlet (12) and an outlet (14) open into the chamber (2) below a screen (20), and an overflow outlet (15) opens into the chamber above the screen. Baffles (26) are provided underneath the screen (20) across the general flow of storm water. In storm conditions, the storm water in the chamber (2) will reach the level of the screen (20), which traps aesthetically offensive material and gross solids, leaving the water to flow through the screen (20) to the overflow outlet (15). The baffles (26) circulate the storm water, and this dislodges material accumulated on the underside of the screen (20) and so helps to prevent blinding of the screen (20) in storm conditions.