ISHTAR (Internal Structure High-resolution Tomography by Asteroid Rendezvous) is a mission developed through ESA General Studies programme. The study, led by Astrium in cooperation with several scientific institutes throughout Europe, has produced a spacecraft design capable of performing multiple asteroid rendezvous and to characterize them with a focussed set of instruments. The ISHTAR concept is centred around a Radar Tomography paylod able to probe the internal structure of a small asteroid to depths of few hundred meters, combined with a small camera for investigation of the surface properties and a radio science experiment for gravity field measurement. This combination will allow the first detailed characterization of a NEO and will give valuable insights into the origin and evolution processes that govern the NEO population. In particular, ISHTAR will be able to visit at least 2 NEOs belonging to two different spectral classes, thereby allowing us to probe the diversity of the NEO population.