EISCAT_3D is a next generation incoherent scatter radar system for high-latitude atmosphere and geospace studies that will be built in northern Fenno-Scandinavia. The facility will consist of multiple large phased-array antenna transmitters/receivers in three countries, comprising tens of thousands of individual antenna elements. The new radars will collect data from the upper stratosphere to the magnetosphere and beyond, contributing to the basic, environmental and applied science that underpins the use of space by contemporary society. EISCAT_3D's capabilities go beyond anything currently available to the international research community and will constitute a valuable scientific resource for the European Research Area.Located in the auroral zone at the edge of the northern polar vortex, EISCAT_3D will provide long-term continuous data for scientists studying global change, measuring the effects of man-made and natural variability on the middle and upper atmosphere. Its observations will underpin space weather prediction and monitoring, essential for the operation and the improved service of European space assets. In addition EISCAT_3D will facilitate studies of solar system influences on the terrestrial environment, such as solar wind, meteors, dust, energetic particles and cosmic rays, in collaboration with other research infrastructures.The Preparatory Phase will resolve the remaining legal, financial and technical questions which must be addressed before the construction of EISCAT_3D.