The Sun's outer atmosphere or corona is heated to millions of degrees, considerably hotter than its cool surface or photosphere. Explanations for this long-standing enigma typically invoke the deposition in the corona of non-thermal energy generated by the interplay of convection and magnetic fields. However, the exact physical mechanism driving coronal heating remains unknown. During the past few years, recently built instruments like the Japanese Hinode satellite, the Swedish Solar Telescope in Spain and NASA's Solar Dynamics Observatory (SDO) combined with advanced numerical simulations have revealed a new window into how the Sun's atmosphere is energized. These results directly challenge current theories and highlight the importance of the interface region between the photosphere and corona for understanding how the solar atmosphere is heated. Dr. De Pontieu will present some of these results and describe how NASA's recently selected Interface Region Imaging Spectrograph, which is being built by Lockheed Martin's Solar and Astrophysics Laboratory in Palo Alto, in collaboration with NASA Ames, Smithsonian Astrophysical Observatory (SAO), Montana State University, Stanford University and the University of Oslo, will be able to address many of the outstanding issues and problems.