Archaeocyathans were shallow marine benthic filter-feeding organisms. They preferred carbonate-rich waters and typically lived in aggregates or communities which were dominated by calcareous algae or algae-like problematica. In addition, a wide range of metazoans either occupied these communities or lived in great abundance nearby. It is now clear that these communities or aggregates were capable of constructing carbonate buildups which were topographically and sedimentologically significant. The Archaeocyatha evolved for some 45 to 50 million years and so did the content and structural complexity of their buildups. This complexity reached its acme during the later part of the Lower Cambrian (late Atdabanian and Lenian of Soviet and European workers). Let us now shift from generalities and broad interpretations to a review of the supportive data.