The Khorassan-Kopet Dagh Mountains are mainly east-west oriented ranges in the northeastern Iran. The climate of the area is continental and distinctly drier than the nearby mountain ranges. A total of 2576 species or infraspecific taxa belonging to 702 genera and 112 families are recorded from these mountains. It is known as a separate floristic province and is a transitional zone between the different phytogeographical units of the Irano-Turanian region. Near to 65% of the species are Irano-Turanian elements, and the level of endemism is about 14%. In spite of the uniform and dry climate of the Khorassan-Kopet Dagh, the heterogeneous topography supported the diverse vegetation types in the region. The major vegetation types of these mountains are Hyrcanian montane forests, Juniper woodlands, Pistacia woodlands, mesophilic shrublands, montane steppe shrublands, subalpine thorn-cushion communities, Stipa-Artemisia steppes, chasmophytic vegetation, edaphic vegetation, halophytic vegetation, psammophyte vegetation, wetlands, and ruderal vegetation. In recent decades, extensive urbanization and industrialization caused dramatic degradation in the natural ecosystems and the biodiversity of the Khorassan-Kopet Dagh. Almost all populations of endemic taxa outside the protected areas are severely threatened and need to be protected.