A filter unit comprises a housing containing a filter element. The housing includes an inlet for fluid and an inlet passage which passes fluid to the filter element. An outlet passage conveys filtered fluid from the element to an outlet to the housing. The inlet passage includes a valve which can close the inlet passage. When this happens, a by-pass valve opens and connects the inlet to the housing direct to the housing outlet. An anti-backflow valve is provided in the outlet passage to prevent fluid passing back down the outlet passage to the filter element. The filter element can then be changed. A re-pressurization valve is provided to pass fluid to the element while the inlet passage is still closed and a vent port vents air from the element during this re-pressurization. Once re-pressurization is complete, the vent port and the re-pressurization valve are closed and the inlet valve opened so that filtration recommences.