A servo valve comprising: comprising: a housing; a supply port, a return port and a control port (P A, P B), defined in the housing; a moveable valve spool body extending in an axial direction located fully within the housing and arranged to allow fluid flow through and out of the moveable valve spool body according to an axial position of the moveable valve spool body relative to the housing; a drive means configured to move the moveable valve spool body in the axial direction relative to the housing in response to a control signal to regulate the fluid flow; and a spring element; wherein the drive means comprises a piezo actuator element located fully within the housing at a first spool end of the movable spool body, the piezo actuator element configured to have a first length defining a neutral position of the moveable valve spool body with respect to the housing and to change its length in the axial direction in response to the control signal to cause corresponding movement in the axial direction of the moveable valve spool body to a position spaced from the neutral position in the axial direction; wherein spring element is located at a second end of the moveable valve spool body opposite the first spool end in the axial direction and is arranged to bias the moveable valve spool body in a direction opposite the axial direction in which the piezo actuator element moves the moveable spool body, wherein in the neutral position, the force of the spring element acting on the second end of the moveable spool body is such as to position the moveable spool body to define flow paths from the oil supply port and the return port and wherein to move the moveable spool body in the axial direction a force is applied by the piezo actuator element on the moveable spool body against the force of the spring element, thus moving the moveable spool body so as to change the flow paths for the hydraulic fluid through the servo valve, characterized in that the piezo actuator element is mounted to the first spool end of the moveable spool body and extends in the axial direction into the moveable spool body from the first spool end. a housing; a supply port, a return port and a control port (P A, P B), defined in the housing; a moveable valve spool body extending in an axial direction located fully within the housing and arranged to allow fluid flow through and out of the moveable valve spool body according to an axial position of the moveable valve spool body relative to the housing; a drive means configured to move the moveable valve spool body in the axial direction relative to the housing in response to a control signal to regulate the fluid flow; and a spring element; wherein the drive means comprises a piezo actuator element located fully within the housing at a first spool end of the movable spool body, the piezo actuator element configured to have a first length defining a neutral position of the moveable valve spool body with respect to the housing and to change its length in the axial direction in response to the control signal to cause corresponding movement in the axial direction of the moveable valve spool body to a position spaced from the neutral position in the axial direction; wherein spring element is located at a second end of the moveable valve spool body opposite the first spool end in the axial direction and is arranged to bias the moveable valve spool body in a direction opposite the axial direction in which the piezo actuator element moves the moveable spool body, wherein in the neutral position, the force of the spring element acting on the second end of the moveable spool body is such as to position the moveable spool body to define flow paths from the oil supply port and the return port and wherein to move the moveable spool body in the axial direction a force is applied by the piezo actuator element on the moveable spool body against the force of the spring element, thus moving the moveable spool body so as to change the flow paths for the hydraulic fluid through the servo valve, characterized in that the piezo actuator element is mounted to the first spool end of the moveable spool body and extends in the axial direction into the moveable spool body from the first spool end.