The geological record contains a fascinating diversity of reefs and shell accumulations. As with many other biosedimentary structures, their facies characterization requires careful observation at outcrop and sample scale, and in thin-section to provide information about the global geometries, fabrics and textures respectively. This collection of papers encompasses the breadth of sedimentary geometries and facies displayed by Palaeozoic reefs, shell accumulations, and transitional composite deposits. The definition of reefs and shell concentrations has given rise to variations in nomenclature. The papers in this volume cover specific problems regarding the nomenclature and facies characterization of reefs, shell accumulations and transitional composite deposits. However, rather than attempt a complete revision of terms, the authors have touched on some of the important issues at this stage of development in the field: the main climatic, environmental and evolutionary factors that controlled the Palaeozoic development of shell accumulations and reefs.