Plankton communities are vital components of aquatic ecosystems, comprising a diverse range of organisms that drift in water columns. They are broadly categorized into phytoplankton, which are photosynthetic, and zooplankton, which are heterotrophic. Phytoplankton, including diatoms, dinoflagellates, and cyanobacteria, form the base of the aquatic food web, converting sunlight and nutrients into organic matter through photosynthesis. This primary production supports a variety of marine and freshwater life forms. Zooplankton, such as copepods, krill, and jellyfish, feed on phytoplankton and other small particles, acting as intermediaries in the transfer of energy from primary producers to larger predators like fish and whales. Plankton communities are highly dynamic and influenced by various environmental factors such as temperature, light availability, nutrient concentrations, and water currents. Seasonal changes and climatic events like El Niño can significantly impact their composition and abundance. The health of plankton communities is crucial for maintaining biodiversity and the overall functioning of aquatic ecosystems. They play a significant role in biogeochemical cycles, particularly in carbon sequestration, as phytoplankton absorb atmospheric CO2 during photosynthesis. This book consists of contributions made by international experts. It will provide interesting topics for research, which interested readers can take up. Students, researchers, experts and all associated with plankton community studies will benefit alike from this book.