The IBM Blue Gene/L computer at LLNL has been at the top of the list of the most powerful supercomputers in the world since November 2004. Harnessing to power of this architecture can be challenging though. The relatively slow processor serving as the building block for this machine forces the user to push the concurrency up by an order of magnitude in order to achieve substantial performance. Tens of thousand of processors are then used for a single simulation. In July 2006, the 3D particle-in-cell code GTC ran a 32,768-processor simulation on the Blue Gene computer at the IBM T.J. Watson Research Center. This talk will explain the steps taken to optimize GTC on BG/L and make this record-size simulation possible.