GRB100116A was detected by both the Large Area Telescope (LAT) and the Gamma-ray Burst Monitor (GBM) onboard the Fermi satellite. The GBM lightcurve showed two peaks, a small precursor and a second, stronger peak about 70 seconds later; the LAT detection was coincident with the second peak. A preliminary spectral fit of the second peak using the GBM data alone resulted in a power law with an exponential cutoff, so that the observed excess in the LAT data is somewhat surprising. In addition, this burst presents some peculiarities in its temporal structure, including the long delay between the LAT (>100 MeV) emission and the GBM trigger. We present a spectral and temporal study of the prompt and afterglow emission from 8 keV to 200 GeV using data from all of the Fermi instruments, including the LAT Low Energy (LLE) events, which proved to be crucial in constraining the spectral fits.