The VIRTIS scientific and technical teams will take advantage of their previous experience in the design and development of spectrometers for space applications. In fact, the various groups contributing to the VIRTIS experiment, from Italy, France and Germany, have been deeply involved in the CASSINI mission, with the experiments VIMS and CIRS. The targets of the ROSETTA mission are the most primitive solar system bodies: comets and asteroids. ROSETTA will study in detail a comet nucleus, the prime target of the mission, and will fly by one or two asteroids. The small bodies of the solar system are of great interest for planetary science and their study is crucial to understand the solar system formation. In fact it is believed that comets and, to a lesser extent, asteroids underwent a moderate evolution so that they preserve some pristine solar system material. Comets and asteroids are in close relationship with the planetesimals, which formed from the solar nebula 4.6 billion years ago. The global characterisation of one comet nucleus and one or two asteroids will provide basic information on the origin of the solar system and on the interrelation between the solar system and the interstellar dust environment. The ROSETTA mission is designed to obtain the above mentioned scientific goals by: (a) in situ analysis of comet material; (b) long period of remote sensing of the comet. The combination of remote sensing and in situ measurements will increase the scientific return of the mission. In fact, the "in situ" measurements will give relevant "ground-truth" for the remote sensing information, and, in turn, the locally collected data will be interpreted in the appropriate scenario provided by remote sensing investigation. The scientific payload of ROSETTA includes a Visual InfraRed Spectral and Thermal Spectrometer (VIRTIS) among the instrument on board the spacecraft orbiting around the comet. This instrument is fundamental to detect and study the evolution of specific fingerprints - such as the typical spectral bands of minerals and molecules - arising from surface components and from materials dispersed in the coma. Their identification is a primary goal of the ROSETTA mission as it will allow us to identify the nature of the main constituent of the comets. Moreover, the surface thermal evolution during comet approach to Sun is important information that can be obtained by means of spectroscopic observation. The VIRTIS design and its detailed science goals are reported hereafter.