An electro-hydrostatic actuator, comprising: a piston assembly having a piston head and a piston rod extending from the piston head; a hydraulic cylinder (30, 210), wherein the piston assembly is located and movable within the hydraulic cylinder, the hydraulic cylinder including a hydraulic fluid chamber region including a piston side chamber and a rod side chamber; a reservoir for storing hydraulic fluid located within the electro-hydrostatic actuator which is in fluid communication with the hydraulic fluid chamber region, wherein the reservoir is formed between an inner surface of a reservoir wall and an outer surface of an outer periphery of the hydraulic cylinder housing, wherein the reservoir has its interior subdivided into a first and a second pressure compartments by a bellow assembly, wherein the first pressure compartment includes a first gas pressure compartment located towards the piston rod end, wherein the second pressure compartment is located towards the piston end and in fluid communication with the hydraulic fluid chamber region and a flow control network, wherein the bellow assembly is a thin-walled metallic or non-metallic bellow and includes an outer bellow and an inner bellow, wherein the outer bellow is fitted to be surrounded by and abut the inner surface of the reservoir wall, while the inner bellow is fitted to surround and abut the outer surface of the hydraulic cylinder housing, the interior of the bellow assembly being a pressure storage chamber filled with a gas under high pressure; a hydraulic pump system for moving hydraulic fluid in the reservoir and the hydraulic fluid chamber region, the hydraulic pump system in fluid communication with the flow control network in a hydraulic cylinder boss for controlling a direction and flow magnitude of hydraulic fluid within the hydraulic fluid chamber region; and an electric motor located adjacent to and operatively coupled to the hydraulic pump system for driving the hydraulic pump system. a piston assembly having a piston head and a piston rod extending from the piston head; a hydraulic cylinder (30, 210), wherein the piston assembly is located and movable within the hydraulic cylinder, the hydraulic cylinder including a hydraulic fluid chamber region including a piston side chamber and a rod side chamber; a reservoir for storing hydraulic fluid located within the electro-hydrostatic actuator which is in fluid communication with the hydraulic fluid chamber region, wherein the reservoir is formed between an inner surface of a reservoir wall and an outer surface of an outer periphery of the hydraulic cylinder housing, wherein the reservoir has its interior subdivided into a first and a second pressure compartments by a bellow assembly, wherein the first pressure compartment includes a first gas pressure compartment located towards the piston rod end, wherein the second pressure compartment is located towards the piston end and in fluid communication with the hydraulic fluid chamber region and a flow control network, wherein the bellow assembly is a thin-walled metallic or non-metallic bellow and includes an outer bellow and an inner bellow, wherein the outer bellow is fitted to be surrounded by and abut the inner surface of the reservoir wall, while the inner bellow is fitted to surround and abut the outer surface of the hydraulic cylinder housing, the interior of the bellow assembly being a pressure storage chamber filled with a gas under high pressure; a hydraulic pump system for moving hydraulic fluid in the reservoir and the hydraulic fluid chamber region, the hydraulic pump system in fluid communication with the flow control network in a hydraulic cylinder boss for controlling a direction and flow magnitude of hydraulic fluid within the hydraulic fluid chamber region; and an electric motor located adjacent to and operatively coupled to the hydraulic pump system for driving the hydraulic pump system.