A dual-fuel engine has a primary fuel supply and a secondary fuel supply, the primary and secondary fuels being arranged in use to mix with each other and with air for combustion in one or more cylinders of the engine. The supply of both the primary and secondary fuels is arranged to be actively managed in accordance with a desired engine performance characteristic by an electronic controller. In one arrangement the supply of a primary fuel is arranged to be actively reduced when a secondary fuel is supplied to the engine.