Dr. Fa Wang, 2009–2012 Pappalardo Fellow (Hard Condensed Matter Theory)In superconductors, electrons form pairs that can transport electricity without dissipation. This pairing requires attraction between electrons while the fundamental interaction between them is Coulomb repulsion. In conventional superconductors, the attraction is provided by the vibration of ions. But this is likely not the case in many unconventional superconductors, including the copper-based and the newly discovered iron-based high Tc superconductors. Understanding the pairing mechanism of these high Tcsuperconductors has become an important subject in condensed matter physics and may help us in the search for new high temperature superconductors. I will briefly review the physics of iron-based high Tc superconductors and describe my related theoretical work, especially my understanding of their pairing mechanism.