Harmful algal blooms (HAB) are defined as an increase in the concentration of a phytoplankton species in a water body that could be triggered by many environmental, meteorological, and hydrological factors simultaneously, such as eutrophication, sea surface temperature, and wind and current directions. HAB issues are global—appearing in locations such as Lake Tai in China, Lake Erie in the United States, and Lake Victoria in Africa—and seem to be increasing in severity and extent under the impact of global changes. These issues result in many unintended environmental, economic, ecological, and human health consequences. They include but are not limited to mass mortalities of fish and marine mammals; economic loss due to reduced tourism, fish stocks, and shellfish harvests; degraded drinking water quality; and a suite of public health problems associated with the consumption of contaminated fish, drinking water, and shellfish via direct/indirect exposure to toxins such as Mycrocystis. These HABs may alter marine/coastal benthic habitats and ecosystem health through shading effect, generate adverse effects on submerged and/or emergent aquatic vegetation, impact various life stages of fish, and cause induced anoxia via overgrowth. Near-surface HABs, like the Karenia spp. blooms, can be easily detected and tracked by space-borne and airborne remote sensing technologies. Yet subsurface HABs, like the Dinophysis spp. blooms, cannot be detected directly with space-borne remote sensing. Both types of HABs, however, can appear as sudden events without early warning and persist in the water body for a period of time.