Phytoplankton, as a primary producer, plays a fundamental role in marine and freshwater ecosystems, and their blooms can have significant ecological, economic, and societal implications. Remote sensing offers tools and methodologies that enable efficient and comprehensive monitoring of phytoplankton dynamics across various spatial and temporal scales. This chapter explores the vast potential of remote-sensing technologies in monitoring phytoplankton blooms. The chapter delves into the capabilities of remote-sensing platforms in capturing the spectral signatures and spatial distribution of phytoplankton in aquatic environments. The discussion encompasses detecting, mapping, and quantifying phytoplankton blooms using remote-sensing data, highlighting the diverse range of sensors and algorithms employed. Moreover, it addresses the role of remote sensing in detecting harmful algal blooms and supporting the development of early-warning systems to mitigate their impacts on marine ecosystems and human health. Finally, it provides valuable insights into current trends, challenges, and future directions in remote-sensing applications for monitoring phytoplankton blooms. Overall, this chapter underscores the critical role of remote sensing in advancing our understanding of phytoplankton dynamics and contributing to the sustainable management of aquatic ecosystems in the face of environmental change and anthropogenic pressures.