Phytoplankton, the microscopic photosynthetic organisms found in aquatic environments, play a vital role in sustaining life on Earth. Despite their minute size, these diverse assemblages of microalgae, cyanobacteria, and other photosynthetic protists contribute significantly to a wide range of ecosystem services that support human well-being and the health of our planet. As primary producers, phytoplankton form the base of aquatic food webs, converting sunlight, carbon dioxide, and nutrients into organic matter, thereby fuelling marine and freshwater ecosystems. Their productivity supports the growth and survival of numerous organisms, from zooplankton and fish to marine mammals and seabirds. Furthermore, phytoplankton contribute substantially to global biogeochemical cycles, regulating the fluxes of essential elements such as carbon, nitrogen, and phosphorus. Beyond their ecological significance, phytoplankton provide invaluable ecosystem services to humanity. They are responsible for approximately half of the world's oxygen production through photosynthesis, sustaining life on Earth. Additionally, certain phytoplankton species are being explored for their potential in carbon sequestration, mitigating the impacts of climate change through the biological capture and storage of atmospheric carbon dioxide. Phytoplankton also plays a role in water purification, as some species can remove pollutants and contaminants from aquatic environments through bioaccumulation or biotransformation processes. Moreover, their diverse metabolic pathways and bioactive compounds hold promise for various biotechnological applications, ranging from biofuel production to pharmaceutical and nutraceutical development. However, anthropogenic activities and environmental changes pose significant threats to phytoplankton communities, with potential cascading effects on the ecosystem services they provide. Climate change, eutrophication, ocean acidification, and other stressors can disrupt phytoplankton dynamics, leading to shifts in community composition, harmful algal blooms, and disruptions in food webs and biogeochemical cycles. This chapter explores the multifaceted roles of phytoplankton in aquatic ecosystems and their contributions to ecosystem services vital for human well-being. It examines the ecological, biogeochemical, and biotechnological significance of these microscopic organisms, while also addressing the potential impacts of environmental stressors on phytoplankton communities and the consequent implications for ecosystem functioning and the services they provide.