Biological diversity refers to all the diversity oflife on earth. The diversity of species is one of the best indicators of biodiversity. Plants are the basis on which depends all the other biodiversity. With 0,075 species/km², Croatia is in the third place, just behind Slovenia and Albania. In Croatia, 8829 species and subspecies of plants and algae have been recorded, but only 86% of the plant world has been researched. Croatia is divided into several biogeographical regions according to the floral element. It stretches on the border of two vegetation regions: Holarktisa and Euro-Siberian-Mediterranean region. In Croatia, there are many habitat types of which some are typical for Croatia. Croatia is rich in endemic flora due to its specific historical, climatic and geomorphological conditions and the large number of habitat types. The main centers of endemic flora arethe mountains Velebit and Biokovo. 2700 plant species and 78 endemic species were discovered on Velebit, of wich the most famous is the Velebit degenia, while mountain Biokovo has about 1.400 species and is one of the endemic centers. The main threat to the Croatian flora are habitat loss or degradation, which are threatened by anthropogenic activities and vegetation succession. One of the causes of vulnerability are invasive plants. In order to preserve the diversity of plants, laws for the protection of certain species are made, listing species in any of the international conventions or protection programs and sites with protected areas. Since 2007 ecologic net mechanism Natura 2000 is used. In 2005 Red Book of Croatian vascular flora was published, containing classified species listed according to the criteria of vulnerability. Red Book can also be searched online at the Flora Croatica Database, as well as the data on the other plant species on the Croatian territory.