Harmful algal bloom(HAB) is an abnormal marine ecological phenomenon, which often associates with disastrous economic or ecological consequences. Typical HAB events include not only red tides or brown tides formed by microalgae, but also green tides formed by some macroalgal species. Once HABs appear in some regions, they would last for a long time and lead to severe impacts on mariculture industry, marine ecosystems, even the health of human-beings. In China, HABs are among the most serious marine ecological problems. Red tides, brown tides, and green tides have been all reported in the coastal waters of China. The Bohai Sea, the Changjiang River estuary and its adjacent waters, and the coastal waters along the South China Sea are identified as three most notable regions for the occurrence of red tides. In the Bohai Sea, large-scale red tides of Gymnodinium sp., Ceratium furca and Phaeocystis sp., as well as the recently identified brown tides of Aureococcus anophagefferens, led to severe impacts on the mariculture industry and huge economic loss. Toxic algal blooms of Dinophysis spp. and Alexandrium spp. were also reported. In the Changjiang River estuary and its adjacent waters, large-scale blooms formed by dinoflagellates Prorocentrum donghaiense, Karenia mikimotoi and Alexandrium spp. started to appear from the beginning of the 21 century and also led to tremendous impacts. A single bloom event of K. mikimotoi along the coast of Fujian Province in 2012 led to economic loss around 2 billion RMB. In the coastal waters of the South China Sea, the intensive blooms of Phaeocystis not only damaged the mariculture industry, but also posed potent threats on the operation of cooling systems of power plant. For the HABs recorded in China, it was also noticed that the bloom-forming species are getting more and more diversified over the last two decades. Besides the microalgal blooms formed by diatoms, dinoflagellates, haptophytes, cyanobacteria, cryptophytes, raphidophytes, and pelagophytes, blooms of macroalgae also occurred widely along the coast of China. More importantly, there are more and more blooms formed by toxic microalgal species, such as Alexandrium spp. and Dinophysis spp. Phycotoxins, such as paralytic shellfish toxins and diarrhetic shellfish toxins, are often detected from phytoplankton and shellfish samples. Besides, the cell size of bloom-forming species are getting smaller. Cell size of some bloom-forming species, such as Phaeocystis spp. and A. anophageffererns, is only about several micrometers, which make it difficult to carry out the routine monitoring based on traditional morphological approaches. The increasing severity of HABs in the coastal waters of China has a close relationship with the intensified eutrophication, especially in the estuaries and bays. Besides, the alteration of natural habitat by mariculture industry and other anthropogenic activities also contributes to the occurrence of some HAB events, such as the green tides in the southern Yellow Sea. Other factors like climate change also influence the distribution, dynamics, and impacts of HABs in the coastal waters of China directly or indirectly, through the effects on water temperature, stratification, and currents. However, such effects are still poorly understood compared to those of land-based anthropogenic activities.