Acoustic Emission (AE0) 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 experimental work described here, by using pencil lead breaks, assesses the suitability of the AE parameters to characterise a source. The work shows that a characterisation must deal with the effects of the material and lay-up, shows the effects that the dimensions of the sample have on the internal reflections of the elastic waves and ultimately on the recorded signals, analyses how the accumulation of the signal with the time can provide useful information, illustrates a compact way to present the typically large number of AE data coming from the testing of composites and shows that the signals coming from a single sensor can carry information on the geometry of the structure. Studies on tensile tests in CFRP strips of different lay-ups and one panel loaded with a four-point bending are also included, to test the performance and the feasibility of AE in charactering actual sources of damage. The novelty of this work consists of the following points: the difference between a 'description' and a 'characterisation' was defined; it was shown that the characteristics of the sensors largely affect the 'description'; the variability introduced by the system and the testing parameters were investigated; 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.