The oceans cover roughly seventy percent of the surface of the earth. With few exceptions minute photosynthetic primary producers like diatoms, dinoflagellates, silicoflagellates and coccolithophores inhabit the upper 200 m of this vast expanse. This phytoplankton forms the base of the marine food chain and plays an important role in geochemical cycles. Knowledge about species level biodiversity and speciation thus is important to understand marine ecology and biogeochemistry. Amongst the mentioned groups coccolithophores appear to be the ideal test group since their biomineralised periplasts the coccoliths provide a rich suite of qualitative and quantitative morphological characters and a uniquely extensive fossil record. In addition to this extant coccolithophore species can be grown in culture and hence are available for molecular genetic studies and cytological research. For the CODENET project (the EC funded Coccolithophorid Evolutionary Biodiversity and Ecology Network) a number of extant species with seemingly global occurrences spanning the biodiversity of coccolithophores have been selected and different case studies have been carried out on them to elucidate their species level biodiversity. Here we present the results and discuss tentative models of speciation for the coccolithophores in general.Following a brief introduction the second chapter (Methods) deals with sampling for both life and fossil coccolithophores and introduce culture techniques as well as biometrical methods used. Chapter 3 (Results) is divided in four parts in part 1-3 detailed results from three CODENET species will be presented, mainly dealing with their morphological variability and physiology, but also with their evolutionary history. Part 4 is a published research paper and highlights the importance of life cycle studies for the interpretation of coccolithophore evolution, diversity and speciation. Chapter 4 (Discussion) comprises a review about species level variation in all the CODENET key species, to be published in a conference proceedings volume by Springer. Further published manuscripts both dealing with methods used in this study, but also supplementary data important for this thesis are given in the Appendix.