Plankton are aquatic organisms unable to swim against the current, and they include diverse taxa of different phylogenetic origins. The taxonomy, phylogeny and ecology of nine plankton groups are reviewed in this paper, in order to comprehensively understand the latest information and current situation of plankton studies. The order-level classification of dinoflagellates was re-arranged, but the classification system is still not well organized at the family-level. The taxonomy of raphidophytes and dictyochophytes was partly confused, however, molecular studies provided clear categorization between these groups. The diatoms could be identified by observing some important morphological characteristics. Yet, these characteristics are sometimes not observable because of inappropriate specimen treatments, and furthermore, the morphological terms are not enough unified, resulting that the species-level identification is complicated and difficult. Recent studies revealed the cryptic diversity and high abundance of some microalgae, such as haptophytes and prasinophytes. The diversity and ecology of planktonic foraminifers have been clarified, but those of radiolarians and phaeodarians are still wrapped in mystery. The classification needs to be re-arranged especially for collodarians, phaeodarians and acantharians. The phylogeny of copepods has been elucidated, and this group was re-classified into 10 orders. Future studies should clarify their evolutionary process and create useful databases for easier identification. The methods to reveal the larva-adult correspondence are established for decapods, and further clarification of the correspondence is expected. The classification system of chaetognaths has been updated, and the intra-species diversity is also being studied. The species diversity of scyphozoans has not been well clarified especially for deep-sea species, and their classification still involves problems such as cryptic species. The dataset including DNA sequences and different types of images (taken in the field and under the microscope, etc.) should be accumulated for comparing the data from different methods (e.g., direct microscopy, optics-based survey and environmental DNA analysis).