Oil, gas and long coastal outfall pipelines laid directly on a sandy seabed can be subject to severe under-scour if the local tidal and wave induced currents are sufficiently strong. Such scour is of both major engineering concern and interest. Free spans, bridging across scour holes, can present local buckling problems and can also be subject to large amplitude, resonant vibration induced by vortex shedding. On the positive side, the scour process can lead to self-burial of the pipelines. The present research programme was carried out to further investigate the mechanics of the self-burial process. Both loose sand beds and rigid flat and scour profiled beds were used in the experimental laboratory work. The effects of different upstream bed conditions which influence the approach flow boundary layer, and different flow reversing times simulating tidal action have been evaluated. It was found that the observed sharp increase in scour depth under the pipeline is uniquely related to the effects of flow reversal and the associated symmetrical pattern of scour development and sediment deposition on either side of the pipeline. The laboratory tests have confirmed that there are three basic mechanisms responsible for initiation and development of scour holes under seabed pipelines, namely, initial breakthrough, tunnel erosion and lee erosion. Vortex shedding in the separated wake flow creates a very wide symmetrical scour hole under reversing flow conditions. This generates significant increases in maximum scour depth compared with uni-directional river type flow. Laser Doppler Anemometer measurements have been used to examine scale effects and have shown that the relative travel distance of the shed vortices and hence the relative scour width and depth increase with decreasing pipeline diameter. Flow visualisation techniques were also employed to gain further insight into the flow regime surrounding the pipeline. In particular the flow pattern inside the model scour hole and around the model pipelines including a newly identified bed jet flow phenomenon producing a vigorous scouring mechanism which is peculiar to reversing flow, have been examined in detail. In spite of considerable efforts made during the past two decades to understand the self-burial mechanism, a comprehensive theoretical model of this complex natural phenomenon has yet to be established. Development of models for prediction of, for example, maximum length of free spans along pipelines and likely self-burial performance, requires detailed knowledge of the three-dimensional growth characteristics and geometry of scour holes. In particular, it is most important to be able to estimate maximum depths of scour. In parallel with the laboratory investigation of the pipeline self-burial mechanism, a theoretical model for estimating maximum scour depth under seabed pipelines has also been developed. Theoretical predictions using this model are in very good agreement with measurements obtained from the specially designed reversing flow facility at UCL. The model further indicates that sagging of pipeline freespans can double maximum scour depths. The findings support the hypothesis that reversing flow, either from tidal or very large relative amplitude wave action, is a necessary condition for deep self-burial of seabed pipelines.