Serpulidae (Polychaeta) are benthic, suspension-feeding worms, mostly marine, secreting their own calcareous habitation tubes. Serpulid 'reefs' include banks deposited on shallow parts of continental shelves, and primary frame reefs at intertidal and subtidal depths. Sheltered bays harbour the largest reefs, up to a few metres in height and kilometres in length. A review of habitats with Recent serpulid mass-occurrences allows interpretation of the palaeoenvironment in which the serpulid limestones in the 'Upper Malm' (Upper Jurassic-Lower Cretaceous) of NW Germany may have been laid down. The Lower Serpulid Limestones represent concentrations of re-deposited serpulid tubes. In the Upper Serpulid Limestones ('Serpulit') the re-deposited tubes are embedded in stromatolitic algae. Both limestones were formed in lagoons fringing an inland sea. Several records of Recent and fossil 'serpulid' buildups are erroneous and can be referred to algae, vermiform 'gastropods' or non-serpulid polychaetes. Differences and similarities between calcareous tubes of serpulids and vermetids (Gastropoda) are summarized.