The first MODerate-resolution Imaging Spectroradiometer (MODIS) was launched on the Terra spacecraft on December 18, 1999. The second MODIS was launched on the Aqua spacecraft on May 4, 2002. As an integral part of NASA's Earth Observing System (EOS), these instruments provide daily observations of nearly the whole Earth with the goal of enhancing scientific characterization of land, ocean, atmosphere, climate change, and natural hazards. MODIS uses a scanning mirror, 490 detectors distributed among 36 spectral bands, and on-board calibrators (a solar diffuser, solar diffuser stability monitor, a blackbody, and a spectro-radiometric calibration assembly) to meet these objectives. The MODIS Level 1B (L1B) algorithms, written in C, input uncalibrated, geo-located observations, convert the instrument response into calibrated data, and generate science data sets. This calibration is performed on a pixel-by-pixel basis for each detector. The instrument characterization needed to run the L1B code is implemented using 96 Terra and 99 Aqua Look-up Tables. This paper describes the current L1B algorithm and discusses the changes made as a consequence of on-orbit analyses and operational considerations.