Cyanobacteria are prokaryotic microorganisms capable of performing photosynthesis and releasing oxygen. They inhabit a wide range of environments, including freshwater, marine ecosystems, moist soils, and other habitats. Although they are often referred to as blue-green algae, they are actually bacteria that possess photosynthetic pigments, enabling them to convert light energy into chemical energy and produce organic compounds essential for the growth and survival of many organisms. Cyanobacteria play a significant role in primary production and in the cycling of carbon and nitrogen within ecosystems. Through photosynthesis, they absorb carbon dioxide and produce organic matter, contributing to the transfer of energy through aquatic food webs. Certain species are also capable of fixing atmospheric nitrogen, converting it into nitrogenous compounds that can be utilized by other organisms. Furthermore, ancient cyanobacteria contributed to major changes in Earth's atmosphere through oxygenic photosynthesis, which played a fundamental role in the gradual accumulation of oxygen in the atmosphere and the evolution of life. Despite their ecological importance, some cyanobacteria can multiply rapidly under favorable environmental conditions, forming what are known as cyanobacterial blooms. These blooms commonly occur in lakes, reservoirs, rivers, and other water bodies when nutrient concentrations, particularly phosphorus and nitrogen, become elevated. Agricultural runoff, untreated or inadequately treated wastewater, and other sources of nutrient pollution can increase the availability of these nutrients. When combined with suitable temperatures, sufficient light, and relatively stable water conditions, these factors may promote excessive cyanobacterial growth. Some cyanobacterial species can produce cyanotoxins, which are harmful substances that may affect human health, animals, and aquatic organisms. Exposure can occur through drinking contaminated water, recreational contact with affected water, or consumption of contaminated food sources. Depending on the type of toxin and the level of exposure, health effects may include skin irritation, gastrointestinal symptoms, liver damage, or neurological effects. However, not all cyanobacterial species produce toxins, and the presence of a bloom does not automatically indicate that toxins are present. Dense cyanobacterial blooms can also disrupt aquatic ecosystems by reducing light penetration, limiting the growth of submerged plants, and altering food-web relationships. When large amounts of cyanobacterial biomass die and decompose, microorganisms consume dissolved oxygen during the decomposition process. This can result in oxygen depletion, potentially causing stress or death in fish and other aquatic organisms. Some blooms may also produce unpleasant odors, reduce water quality, and interfere with the use of water for drinking, recreation, and other purposes. The management of cyanobacterial blooms requires regular monitoring of water quality, nutrient concentrations, and the types of cyanobacteria present. Laboratory identification and appropriate toxin testing are important because water color and visual appearance alone cannot reliably determine whether a bloom is toxic. Reducing the entry of excess nitrogen and phosphorus into water bodies, improving wastewater treatment, and managing agricultural runoff are essential measures for preventing excessive growth and protecting aquatic ecosystems and public health. Al-Mustaqbal University – The First University in Iraq