Plankton (phytoplankton, zooplankton) form the base of the food web. Inflow variability modulates plankton concentration, community composition, and productivity in estuaries. This chapter synthesizes data on plankton and their relationship with inflow from individual bay systems along the Texas coast where sufficient data exists. This will be primarily the Trinity-San Jacinto Estuary (also known as Galveston Bay) and Baffin Bay (located in the Upper Laguna Madre complex) with some details for San Antonio Bay, Mission Aransas Estuary, and Nueces-Corpus Christi Bay. In the former two systems, there is a lot of knowledge on key harmful algal bloom-forming taxa. This chapter will also highlight deficiencies in our current knowledge and future data needs. Given the general lack of data on zooplankton in Texas estuaries, this part of chapter is descriptive in nature. We end with providing a synthesis and recommendation for future efforts.