A method of controlling a length of an air gap in an electric machine using an air gap controller, the electric machine comprising: a rotor core assembly comprising a rotor core and a rotor shaft operably coupled to the rotor core; a stator core assembly comprising a stator core and a stator housing operably coupled to the stator core, the stator core circumferentially surrounding the rotor core; an air gap having a length, L, the air gap located between and defined by an inner surface of the stator core and an outer surface of the rotor core; and a coolant circulation system, the coolant circulation system comprising a cooling conduit defining a pathway for circulating coolant through the electric machine and/or an air conduit defining a pathway for supplying cooling air to the electric machine; the method comprising: determining an air gap length value for the electric machine at least in part using an air gap controller; comparing the determined air gap length value to an air gap target value using the air gap controller; and outputting one or more control commands from the air gap controller to one or more controllable devices,,,,, associated with an air gap control system when the determined air gap length value differs from the air gap target value by a predefined threshold; wherein the one or more control commands are configured to impart a change to one or more operating parameters associated with the air gap control system to adjust a length of an air gap between an outer surface of a rotor core and an inner surface of a stator core of the electric machine; characterised in that the one or more operating parameters that are changed to adjust a length of the air gap include one or more of: a temperature of coolant flowing through the cooling conduit; a flow rate of coolant flowing through the cooling conduit; or a temperature of cooling air flowing through the air conduit. a rotor core assembly comprising a rotor core and a rotor shaft operably coupled to the rotor core; a stator core assembly comprising a stator core and a stator housing operably coupled to the stator core, the stator core circumferentially surrounding the rotor core; an air gap having a length, L, the air gap located between and defined by an inner surface of the stator core and an outer surface of the rotor core; and a coolant circulation system, the coolant circulation system comprising a cooling conduit defining a pathway for circulating coolant through the electric machine and/or an air conduit defining a pathway for supplying cooling air to the electric machine; the method comprising: determining an air gap length value for the electric machine at least in part using an air gap controller; comparing the determined air gap length value to an air gap target value using the air gap controller; and outputting one or more control commands from the air gap controller to one or more controllable devices,,,,, associated with an air gap control system when the determined air gap length value differs from the air gap target value by a predefined threshold; wherein the one or more control commands are configured to impart a change to one or more operating parameters associated with the air gap control system to adjust a length of an air gap between an outer surface of a rotor core and an inner surface of a stator core of the electric machine; characterised in that the one or more operating parameters that are changed to adjust a length of the air gap include one or more of: a temperature of coolant flowing through the cooling conduit; a flow rate of coolant flowing through the cooling conduit; or a temperature of cooling air flowing through the air conduit. determining an air gap length value for the electric machine at least in part using an air gap controller; comparing the determined air gap length value to an air gap target value using the air gap controller; and outputting one or more control commands from the air gap controller to one or more controllable devices,,,,, associated with an air gap control system when the determined air gap length value differs from the air gap target value by a predefined threshold; wherein the one or more control commands are configured to impart a change to one or more operating parameters associated with the air gap control system to adjust a length of an air gap between an outer surface of a rotor core and an inner surface of a stator core of the electric machine; characterised in that the one or more operating parameters that are changed to adjust a length of the air gap include one or more of: a temperature of coolant flowing through the cooling conduit; a flow rate of coolant flowing through the cooling conduit; or a temperature of cooling air flowing through the air conduit.