  The Fermi Large Area Telescope (Fermi-LAT, hereafter LAT), the primary
instrument on the Fermi Gamma-ray Space Telescope (Fermi) mission, is an
imaging, wide field-of-view, high-energy \gamma-ray telescope, covering the
energy range from 20 MeV to more than 300 GeV. During the first years of the
mission the LAT team has gained considerable insight into the in-flight
performance of the instrument. Accordingly, we have updated the analysis used
to reduce LAT data for public release as well as the Instrument Response
Functions (IRFs), the description of the instrument performance provided for
data analysis. In this paper we describe the effects that motivated these
updates. Furthermore, we discuss how we originally derived IRFs from Monte
Carlo simulations and later corrected those IRFs for discrepancies observed
between flight and simulated data. We also give details of the validations
performed using flight data and quantify the residual uncertainties in the
IRFs. Finally, we describe techniques the LAT team has developed to propagate
those uncertainties into estimates of the systematic errors on common
measurements such as fluxes and spectra of astrophysical sources.
