The present book is designed for acquiring a basic knowledge on marine and estuarine phytoplankton and zooplankton, which are tiny free-floating biotic components of the aquatic ecosystem. These drifting biological chips of the aquatic system play an important role in maintaining the global carbon cycle and also in spinning the marine food web. It is well known that phytoplankton are the source of fossil fuels, and form the foundation stone of world fishery. However, phytoplankton sometimes causes adverse impact on the marine and estuarine environments. Toxic phytoplankton, when consumed by shellfish or other species often affect the marine food chain including poisoning of sea birds, mammals and even human. Recent internet reference (http: //oceanlink.island.net/ask/pollution.html) has revealed that phytoplankton into the atmosphere. This compound can transform into sulphuric acid and eventually may contribute to acid rain. The zooplankton occupy the tier next to phytoplankton in the marine food web and form important linkage between the primary and secondary production of the oceans. The present book has focussed on the habitat, taxonomy and ecological role played by the planktonic community in the marine and estuarine compartments. This book can enrich the knowledge of the students on marine ecosystem, the process of sampling, preservation and diversity study of planktonic species in the saline environment. Some case studies have been given highlighting the diversity of plankton in the northwestern Bay of Bengal. Annotated suggested references and internet references have been added at the back of each chapter for those students interested in pursuing further information on this particular branch of environmental science.