When the principles of Sequence Stratigraphy were first published 20 years ago, it was not immediately clear that this concept would revolutionise the way we look at deposition and architecture of sedimentary rocks. Perhaps in retrospect it should not have been so surprising. For the first time seismic data were clear enough that large scale depositional geometries could be resolved; geometries that were not evident from well data alone, and in outcrop work visible only in the largest cliff sections. The observations from seismic data made by the Exxon workers in the 1960's and 1970's were a crucial "piece of the jigsaw" in our knowledge of the way sediments are deposited, and formed the basis for the new paradigm of Sequence Stratigraphy.Gradually through the 1980's the tool of Sequence Stratigraphy was applied to a wide variety of subsurface problems; most commonly large-scale regional reviews of 2D seismic data. Geologists and geophysicists in the oil industry began to realise that here was a way of thinking about rocks that could be used in a true predictive sense. The paradigm implied, that one systems tract should follow another in a predictable way, that observations in one part of a basin had implications in another part, and that undetected play systems could be inferred, and targeted with exploration programs.Sequence Stratigraphy has now gone through a second phase of evolution. The initial concepts have been applied to well and core data. Methodologies have evolved for identifying systems tracts from trends in logs and depositional facies, and from (often subtle) observations in core. The resolving power of 3D seismic data has increased considerably, and we can now see depositional bodies on a relatively small scale, and map their internal character using attribute analysis. Sequence Stratigraphy has entered its High Resolution phase.