A fuel system comprises a pressure raising valve (28) operable to control communication between a fuel delivery line (18) and an engine manifold line (30), and an ecology valve (42) comprising a piston (44) movable within a bore (46), the piston (44) and bore (46) together defining a control chamber (48) and an ecology chamber (50), the ecology chamber (50) communicating with the engine manifold line (30), the pressure within the control chamber (48) being controllable to control the position occupied by the piston (44), the ecology valve (42) further comprising seal means 64) whereby, when the piston (44) occupies a first position in which it engages the seal means (64), only a first, relatively small, effective area of the piston (44) is exposed to a fuel pressure within the engine manifold line (30), a second, larger, effective area being exposed to the fuel pressure within the engine manifold line (30) when the piston (44) occupies a position in which it does not engage the seal (64). An ecology valve (42) for use in the fuel system is also described.