The present invention provides a booster apparatus (10) for entraining gas in a flowing second fluid. The booster apparatus comprises a booster housing (116) for receiving a fluid. The booster apparatus has at least one inlet (123) through which a first fluid passes to be entrained in the second fluid when the second fluid is flowing through the booster housing. The present invention also provides a booster assembly (12) comprising a booster apparatus (10) and a fluid motive mechanism such as a turbine unit (11).