A refrigeration system comprising: a refrigeration circuit that includes at least a compressor, a condenser, an expansion valve and an evaporator; a variable-speed condenser fan; and a controller configured to, in a first mode of operation: monitor a set of operating parameters of the refrigeration circuit that is indicative of a refrigeration demand of the refrigeration system using a plurality of sensors (60, 62, 64, 66, 68, 83); perform an optimisation algorithm using a predetermined model of system efficiency that is a function of the set of operating parameters and condenser fan speed to determine a speed of the condenser fan at which system efficiency will be at its maximum for the current values of the set of operating parameters; and set a speed of the condenser fan based on the determined speed of the condenser fan at which system efficiency will be at its maximum for the current values of the set of operating parameters, wherein performing the optimisation algorithm comprises: for each condenser fan speed of a group of predefined condenser fan speeds: inputting the condenser fan speed in question and the current operating parameter values into the predetermined model of system efficiency; receiving as output data from the predetermined model of system efficiency a refrigeration capacity value and a fuel or power consumption value; and determining, based on the output data for the group of predefined condenser fan speeds, the predefined condenser fan speed for which the corresponding refrigeration capacity value and fuel or power consumption value yields the greatest system efficiency. a refrigeration circuit that includes at least a compressor, a condenser, an expansion valve and an evaporator; a variable-speed condenser fan; and a controller configured to, in a first mode of operation: monitor a set of operating parameters of the refrigeration circuit that is indicative of a refrigeration demand of the refrigeration system using a plurality of sensors (60, 62, 64, 66, 68, 83); perform an optimisation algorithm using a predetermined model of system efficiency that is a function of the set of operating parameters and condenser fan speed to determine a speed of the condenser fan at which system efficiency will be at its maximum for the current values of the set of operating parameters; and set a speed of the condenser fan based on the determined speed of the condenser fan at which system efficiency will be at its maximum for the current values of the set of operating parameters, wherein performing the optimisation algorithm comprises: for each condenser fan speed of a group of predefined condenser fan speeds: inputting the condenser fan speed in question and the current operating parameter values into the predetermined model of system efficiency; receiving as output data from the predetermined model of system efficiency a refrigeration capacity value and a fuel or power consumption value; and determining, based on the output data for the group of predefined condenser fan speeds, the predefined condenser fan speed for which the corresponding refrigeration capacity value and fuel or power consumption value yields the greatest system efficiency. monitor a set of operating parameters of the refrigeration circuit that is indicative of a refrigeration demand of the refrigeration system using a plurality of sensors (60, 62, 64, 66, 68, 83); perform an optimisation algorithm using a predetermined model of system efficiency that is a function of the set of operating parameters and condenser fan speed to determine a speed of the condenser fan at which system efficiency will be at its maximum for the current values of the set of operating parameters; and set a speed of the condenser fan based on the determined speed of the condenser fan at which system efficiency will be at its maximum for the current values of the set of operating parameters, wherein performing the optimisation algorithm comprises: for each condenser fan speed of a group of predefined condenser fan speeds: inputting the condenser fan speed in question and the current operating parameter values into the predetermined model of system efficiency; receiving as output data from the predetermined model of system efficiency a refrigeration capacity value and a fuel or power consumption value; and determining, based on the output data for the group of predefined condenser fan speeds, the predefined condenser fan speed for which the corresponding refrigeration capacity value and fuel or power consumption value yields the greatest system efficiency. inputting the condenser fan speed in question and the current operating parameter values into the predetermined model of system efficiency; receiving as output data from the predetermined model of system efficiency a refrigeration capacity value and a fuel or power consumption value; and determining, based on the output data for the group of predefined condenser fan speeds, the predefined condenser fan speed for which the corresponding refrigeration capacity value and fuel or power consumption value yields the greatest system efficiency.