Environmental alteration, increase of earth’s temperature, different calamities like cyclonic disaster, biotic pressures, Invasive Alien Species (IAS), deforestation, pollution, rainfall pattern, landslides, cloudburst, thunderstorm etc. affecting the ecosystems and native species across the world. IAS is one of the serious warning to biodiversity and considered the main cause of habitat destruction. An invasive species particularly of forest ecosystem is referred as Forest Invasive Species (FIS). FIS is directly related to the forestry and serious threat to forest cover. FIS can remove native species, food and forest cover. Human trade, tourism, transport and travel have enhanced the spread of FIS. For example, Lantana has invaded forest lands, Parthenium in agricultural and forestry areas and Eupatorium in mountain ecosystems, etc. FIS is highly significant in the national context. The bioclimatic conditions of India have been congenial for the alien species to invade in to the various landscapes and ecosystems. There is a lacuna in the knowledge on the biology, geographical distribution, genetic diversity and inter-crossing (hybridizing) potentials of the forest invasive alien species. The entry of eucalyptus gall insect, Leptocybe invasa, in different countries, including India has threatened Eucalyptus species throughout the world. The present chapter has contemplated and described succinctly the IAS with special emphasis on FIS in India. Further, it also highlights the impacts of FIS in forestry and forest cover, their prevention, current position, measures to control FIS, categorization of invasiveness, environmental impact assessment, possible management of FIS to save native forests as well as environment and finally the future scope of research.