GRB 090902B is a luminous, long gamma-ray burst that displays the full range of behaviors that have become hallmark features of the more interesting LAT-detected bursts. During the prompt phase, a significant power-law component extending from the lowest energies (<100 MeV) is detected in addition to the usual Band function component. This power-law component defies easy explanation by synchrotron self-Compton models of GRB emission. In addition, this burst shows declining power-law emission in the LAT band that persists well after the prompt phase has ended and which lasts as long as 1 ks post-trigger. Although it is tempting to attribute this extended emission to the standard afterglow mechanisms (i.e., external shocks), it may also be the product of continued activity by the GRB central engine. In this presentation, we describe the complex spectral evolution and the constraints on the variability .me scales during the prompt and extended emission phases, thereby setting the observational context for the theoretical interpretation and modeling of these phenomena.