  We present MultiModeCode, a Fortran 95/2000 package for the numerical
exploration of multifield inflation models. This program facilitates efficient
Monte Carlo sampling of prior probabilities for inflationary model parameters
and initial conditions and is the first publicly available code that can
efficiently generate large sample-sets for inflation models with $\mathcal
O(100)$ fields. The code numerically solves the equations of motion for the
background and first-order perturbations of multi-field inflation models with
canonical kinetic terms and arbitrary potentials, providing the adiabatic,
isocurvature, and tensor power spectra at the end of inflation. For models with
sum-separable potentials MultiModeCode also computes the slow-roll prediction
via the $\delta N$ formalism for easy model exploration and validation. We pay
particular attention to the isocurvature perturbations as the system approaches
the adiabatic limit, showing how to avoid numerical instabilities that affect
some other approaches to this problem. We demonstrate the use of MultiModeCode
by exploring a few toy models. Finally, we give a concise review of multifield
perturbation theory and a user's manual for the program.
