Acoustic Emission (AE) is a well established Non- Destructive Testing and Evaluation technique for damage monitoring and flaw location and has been used in a wide variety of fields, such as the aerospace and nuclear industry. The material studied here is fibre reinforced composites, which are not monolithic and therefore can fail in different modes. The research determines whether a characterisation of the damage is possible in terms of AE. This is accomplished by studying classic AE features (such as duration, counts, etc) and the frequency content. The novelty of this work consists of the following points: 1) The difference between a description and a characterisation was defined. 2) It was shown that the characteristics of the sensors largely affect the description. 3) The variability introduced by the system and the testing parameters were investigated. 4) The importance of the non-stationarity of the signals was illustrated, together with how this can yield to new information. It was concluded that a characterisation can only have a weak meaning.