ABSTRACT: To help understand potential biases in coccolithophore paleo-biodiversity estimates various aspects of their biology are reviewed based on the recent monographic guide to extant coccolithophore taxonomy and research developed within the CODENET project. The evidence for life-cycles in coccolithophores involving two biomineralized phases is reviewed in detail, and it is shown that these are likely to have been pervasive through the fossil record of coccolithophores. The extent of polymorphism in extant coccolithophores is reviewed and a surprisingly strong correlation with phylogeny is documented, in particular it is shown that there is a very strong correlation between polymorphism in heterococcolith and holococcolith-bearing phases. Recent documentation of (pseudo)cryptic speciation in coccolithophores is discussed and it is argued that this provides considerable support for the rather fine morphological taxonomy adopted by nannofossil paleontologists, but also means that accurate recognition of species in the fossil record is an almost impossible goal. Finally the diversity of extant coccolithophores is compared with that recorded in Holocene sediments and it is shown that the majority of extant diversity is represented by small rare species with very low preservation potential. It is argued that this is the most serious potential biasing mechanism for study of paleobiodiversity changes since estimates of diversity at any time will be strongly influenced by the availability of well-preserved shelf sediments and of the intensity of their study by electron microscopy. Moreover, secular changes in coccolith size in relation to environmental change could significantly affect the preserved diversity of coccolithophores.