This study has demonstrated that the phenomenon of acoustic emission can be used to detect and monitor first time deformation characteristics in stiff, heavily-overconsolidated clay soils. The results from two field studies are presented and discussed with the aim of identifying the mechanisms responsible for the generation of acoustic emission. The first study, conducted at the Building Research Establishment's test site at Cowden (Humberside), was a section of cliff, naturally eroded by sea action. The second study, conducted in a 'moth-balled' brick-pit in Arlesey (Bedfordshire), was a large scale test in which instability was induced in a cut slope (in the Gault Clay) by cutting away at the toe. A number of different designs of wave guide were installed at both sites in order to determine the relative performance of each and, also, to provide further insight into the generation mechanisms. Some consideration has also been given to the effect of alternative data processing techniques. The results obtained indicate that the active wave guide model is the more appropriate in explaining the occurrence of acoustic emission and this finding is supported by the results of laboratory tests designed to replicate the perceived role of the wave guide in the field. In itself, identification of the generation mechanism is a result but, more importantly, understanding of the mechanics enables the design of monitoring systems which are suited to particular geotechnical conditions. Some comparison is also made between acoustic emission and standard methods of deformation monitoring. It is observed that acoustic emission can detect and clearly indicate disturbances at the same time as standard methods record such small magnitudes of movement as to be indistinguishable from instrument error. Additionally, a number of triaxial shear tests were conducted on a selection of soils which illustrates that different soils do exhibit different acoustic characteristics. Finally, this study demonstrates the feasibility of envelope processing using analogue to digital signal conversion both in the field and laboratory. Four published papers pertaining to work described within this thesis are appended.