An active antenna test system comprising: an active antenna unit comprising: a test signal generator arranged to generate at least a first test signal and at least one second test signal; a plurality of transmitter modules (207, 240) operably coupled to the test signal generator wherein the plurality of transmitter modules are arranged to simultaneously process the first test signal and at least one second test signal to produce at least one radio frequency test signal therefrom; at least one receiver module (249, 261) arranged to process one or more signals falling in at least one spectral band determined to be susceptible to intermodulation distortion products caused by the at least one radio frequency test signal being generated from the first test signal and at least one second test signal; and an intermodulation determination module operably coupled to the at least one receiver module and arranged to determine a first received intermodulation performance, wherein the test system is configured to carry out the following process: a first transmitter module of the plurality of transmitter modules is operably uncoupled from the test signal generator and at least a first test signal and at least one second test signal re-applied to the remaining transmitter modules, such that the intermodulation determination module determines a second received intermodulation performance in order to determine an intermodulation distortion contribution of the first transmitter module therefrom. an active antenna unit comprising: a test signal generator arranged to generate at least a first test signal and at least one second test signal; a plurality of transmitter modules (207, 240) operably coupled to the test signal generator wherein the plurality of transmitter modules are arranged to simultaneously process the first test signal and at least one second test signal to produce at least one radio frequency test signal therefrom; at least one receiver module (249, 261) arranged to process one or more signals falling in at least one spectral band determined to be susceptible to intermodulation distortion products caused by the at least one radio frequency test signal being generated from the first test signal and at least one second test signal; and an intermodulation determination module operably coupled to the at least one receiver module and arranged to determine a first received intermodulation performance, a test signal generator arranged to generate at least a first test signal and at least one second test signal; a plurality of transmitter modules (207, 240) operably coupled to the test signal generator wherein the plurality of transmitter modules are arranged to simultaneously process the first test signal and at least one second test signal to produce at least one radio frequency test signal therefrom; at least one receiver module (249, 261) arranged to process one or more signals falling in at least one spectral band determined to be susceptible to intermodulation distortion products caused by the at least one radio frequency test signal being generated from the first test signal and at least one second test signal; and an intermodulation determination module operably coupled to the at least one receiver module and arranged to determine a first received intermodulation performance, wherein the test system is configured to carry out the following process: a first transmitter module of the plurality of transmitter modules is operably uncoupled from the test signal generator and at least a first test signal and at least one second test signal re-applied to the remaining transmitter modules, such that the intermodulation determination module determines a second received intermodulation performance in order to determine an intermodulation distortion contribution of the first transmitter module therefrom.