Observations at 1 AU show that ions can be accelerated up to several GeVs and electrons to tens of MeV in large solar energetic particle events (SEPs). Large SEP events are associated with both large solar flares and fast Coronal Mass Ejections (CMEs), but they are believed to be mainly due to acceleration by CME-driven shocks that propagate through the heliosphere. Determining the origins of SEPs is important because they cause large radiation increase in interplanetary space and over the Earth's polar regions, and because they provide an opportunity to understand the background physics of energetic particle acceleration, escape and transport along the interplanetary magnetic field.Solar Orbiter is an ESA mission (launch currently foreseen for 2015) conceived to perform by first time a close-up study of the Sun (maximum approach 48 solar radii) and of the inner heliosphere, from an out-of-the ecliptic (~30 degrees) vantage point. The spacecraft will carry a set of instruments to observe the solar atmosphere (UV and visible telescopes, coronographs and a magnetograph), as well as charged particles, magnetic field and wave detectors for in situ measurements of particle fluxes, solar wind plasma and the interplanetary magnetic field.