Algal blooms are mass accumulations of cells of one or more phytoplankton species that arise when growth rates (controlled by resource supply, i.e., bottom-up factors) exceed mortality rates (controlled by pathogens, parasitoids, predators, i.e., top-down factors). The species with the highest growth and lowest mortality rates accumulate the most biomass, the upper level of which is set by the limiting resource (light, macronutrients, or iron). Phytoplankton blooms are generally dominated by comparatively few unrelated genera of diatoms with dinoflagellates, haptophytes, and cyanobacteria next in importance. Blooms overgrow the ubiquitous recycling or regenerating communities based on the microbial food web that characterize pelagic ecosystems. A portion of bloom biomass is retained within the surface food web by grazing and cell mortality due to viral infection, the remainder sinks out as whole cells, and phytodetritus is exported to deeper layers and the benthos. Hence, blooms are of great significance for surface and deep food webs as well as for biogeochemical element cycling. Harmful algal blooms (HABs) that cause fish kills or render sea food toxic for human consumption are of economic importance. Despite our growing knowledge of phytoplankton taxonomy/phylogeny, ecology, physiology, and functional genetics, a mechanistic understanding of species dominance in phytoplankton blooms is still lacking.