Marine plankton, despite their tiny size, are crucial for the functioning of marine food webs. It is not just about one organism eating another; when this happens, it helps release the nutrients accumulated within living matter, making them available again for microalgae. Plankton are present in freshwater and in all the seas and oceans on the planet. Moreover, they are responsible for supporting life on Earth and has provided us (on geological timescales) with a significant portion of the oxygen we breathe. In just a small spoonful of seawater, we can find around fifty million viruses, five million bacteria, hundreds of thousands of small unicellular flagellates, thousands of microscopic algae, five heterotrophic ciliates or dinoflagellates, and, with some luck, a small crustacean like copepods. This book aims to bridge the gap between scientific research and the general public, offering a captivating exploration of the fascinating world of marine plankton. Through engaging narratives and visually compelling illustrations, readers will gain insights into the importance of plankton in marine ecosystems, their diverse forms, and the crucial role they play in maintaining our planet's health.