A refrigeration system for a transport refrigeration unit for cooling a temperature controlled cargo space, said refrigeration system comprising: a primary refrigerant circuit including a refrigerant compression device, a refrigerant heat rejection heat exchanger downstream of said compression device, a refrigerant heat absorption heat exchanger downstream of said refrigerant heat rejection heat exchanger, and a primary expansion device disposed downstream of said refrigerant heat rejection heat exchanger and upstream of said refrigerant heat absorption heat exchanger; a motor operatively associated with said compression device for driving said compression device; a variable speed drive operatively associated with said motor for varying the speed of operation of said compression device; and a controller operatively associated with said variable speed drive and said compression device for controlling a cooling capacity of the refrigeration system by selectively controlling the speed of said compression device; characterised in that said controller is configured to operate the variable speed drive in three modes of operation: a pull down mode in which the temperature controlled cargo space is at a higher temperature than a desired temperature and the controller is configured to cool the temperature controlled cargo space as rapidly as possible using the maximum cooling capacity of the system by operating the compression device in a continuous run mode; a capacity mode of operation in which the controller is operable to maintain the temperature in the temperature controlled cargo space within a temperature range by operating the compression device in a power on/off cycling mode at a fixed speed; a part load mode of operation, in which the controller is operable to operate the compression device at a cooling capacity that is less than a maximum cooling capacity by reducing the speed of the compression device to a speed less than the a maximum speed of the compression device, the controller being configured to operate the compression device in a continuous run, variable speed mode in a first portion of the part load mode and a power on/off cycling run, fixed speed mode in a second portion of the part load mode. a primary refrigerant circuit including a refrigerant compression device, a refrigerant heat rejection heat exchanger downstream of said compression device, a refrigerant heat absorption heat exchanger downstream of said refrigerant heat rejection heat exchanger, and a primary expansion device disposed downstream of said refrigerant heat rejection heat exchanger and upstream of said refrigerant heat absorption heat exchanger; a motor operatively associated with said compression device for driving said compression device; a variable speed drive operatively associated with said motor for varying the speed of operation of said compression device; and a controller operatively associated with said variable speed drive and said compression device for controlling a cooling capacity of the refrigeration system by selectively controlling the speed of said compression device; a pull down mode in which the temperature controlled cargo space is at a higher temperature than a desired temperature and the controller is configured to cool the temperature controlled cargo space as rapidly as possible using the maximum cooling capacity of the system by operating the compression device in a continuous run mode; a capacity mode of operation in which the controller is operable to maintain the temperature in the temperature controlled cargo space within a temperature range by operating the compression device in a power on/off cycling mode at a fixed speed; a part load mode of operation, in which the controller is operable to operate the compression device at a cooling capacity that is less than a maximum cooling capacity by reducing the speed of the compression device to a speed less than the a maximum speed of the compression device, the controller being configured to operate the compression device in a continuous run, variable speed mode in a first portion of the part load mode and a power on/off cycling run, fixed speed mode in a second portion of the part load mode.