  Cosmic Reionization On Computers (CROC) is a long-term program of numerical
simulations of cosmic reionization. Its goal is to model fully
self-consistently (albeit not necessarily from the first principles) all
relevant physics, from radiative transfer to gas dynamics and star formation,
in simulation volumes of up to 100 comoving Mpc, and with spatial resolution
approaching 100 pc in physical units. In this method paper we describe our
numerical method, the design of simulations, and the calibration of numerical
parameters. Using several sets (ensembles) of simulations in 20 Mpc/h and 40
Mpc/h boxes with spatial resolution reaching 125 pc at z=6, we are able to
match the observed galaxy UV luminosity functions at all redshifts between 6
and 10, as well as obtain reasonable agreement with the observational
measurements of the Gunn-Peterson optical depth at z<6.
