The study of the diversity and composition of marine communities is the first step in understanding the development of marine ecosystems. While some cnidarian populations are in decline, others invade new regions and habitats. The pressure of the human actions on the marine ecosystems has increased in the past decades. The artificial structures in marine environments, shipping, aquaculture, global warming and interoceanic canals have contributed to the dispersion, establishment and invasion of many places by alien species around the world, in some cases, with major ecological and socio-economic impacts. Some cnidarian species may serve as examples of the truly widespread reach of invasive species. Benthic phase of hydroids and anthozoans are common components of harbours and fouling communities and, probably, they have been transported on ship hulls. Ephyrae and hydromedusae are also frequently found in ballast water. Additionally, the global climatic change allows cnidarian of tropical affinity to extend their range into the temperate zones. The biological invasions and range extensions may cause local native biodiversity and economic losses. Only recently have these biological invasions attracted the attention of the scientific community. Though invasive alien cnidarians are recognized now as a global threat to biodiversity, and monitoring their presence and impacts is considered a prerequisite for marine environmental management and sustainable development, it seldom takes place even in the coastal regions most vulnerable to introductions