The microscopic plants and animals of our oceans known as plankton fix 100 million tons of CO2 per day, produce half the oxygen we breathe, support almost all marine life, impact human health through harmful algal blooms, contribute to cloud formation, and help to move carbon into the deep ocean. Because of these critical ecosystem roles and the sensitivity of plankton to their environment, impacts of climate change on plankton reverberate throughout marine ecosystems. Plankton have exhibited some of the largest range shifts in response to global warming of any marine or terrestrial group, expanding their distribution poleward as temperatures warm. There have also been striking examples of changes in plankton abundance as waters warm, with warm-water species increasing in abundance and replacing cold-water forms. Changes in phenology, or the timing of repeated seasonal activities, are greater for plankton than for terrestrial plants and animals. The timing of phytoplankton blooms appears to have advanced more than for zooplankton, which may disrupt the synchrony between primary and secondary production, leading to inefficient transfer of energy to fish. The increased concentration of CO2 in our oceans is turning them more acidic, potentially making it more difficult for some plankton to build and maintain their calcium carbonate shells. Observations and modeling work suggest that the large tropical oceans are becoming warmer and morestratified, with fewer nutrients in surface waters, leading to smaller phytoplankton cells. As more trophic linkages are needed to transfer energy from small phytoplankton to higher trophic levels, this ultimately results in fewer fish, marine mammals, and seabirds. This could also cause less carbon to be drawn down into the deep ocean, leading to less CO2 diffusing from the atmosphere into the oceans, and elevated atmospheric concentrations. Ultimately, impacts of climate change on plankton will not only determine the future of marine ecosystems but will also influence the pace and extent of climate change globally.