  In this paper we introduce Enzo, a 3D MPI-parallel Eulerian block-structured
adaptive mesh refinement cosmology code. Enzo is designed to simulate
cosmological structure formation, but can also be used to simulate a wide range
of astrophysical situations. Enzo solves dark matter N-body dynamics using the
particle-mesh technique. The Poisson equation is solved using a combination of
fast fourier transform (on a periodic root grid) and multigrid techniques (on
non-periodic subgrids). Euler's equations of hydrodynamics are solved using a
modified version of the piecewise parabolic method. Several additional physics
packages are implemented in the code, including several varieties of radiative
cooling, a metagalactic ultraviolet background, and prescriptions for star
formation and feedback. We also show results illustrating properties of the
adaptive mesh portion of the code. Enzo is publicly available and can be
downloaded at http://cosmos.ucsd.edu/enzo/ .
