HACC (Hardware/Hybrid Accelerated Cosmology Code) is an extreme-scale cosmological simulation code that runs on all available supercomputing platforms at very high performance levels (Gordon Bell Award Finalist 2012, 2013). The origins of HACC lie in a new code design effort initiated for Roadrunner at Los Alamos National Laboratory, the first machine to break the petaflop barrier in 2008. HACC uses a hybrid algorithm in its gravity solver, with the short-range computation being tuned to the system architecture. More about the gravity-only version of HACC can be found in this publication. Gasdynamics in HACC is treated using CRK-SPH (Conservative Reproducing Kernel Smoothed Particle Hydrodynamics), a higher-order SPH scheme that does not suffer from difficulties in dealing with mixing and fluid instabilities. A number of subgrid models for gas cooling/heating, star formation, and astrophysical feedback mechanisms are included. Some of the world’s largest cosmological simulation runs and large-scale simulation suites have been carried out with HACC. Current code development is led by an Argonne team and is supported by DOE’s Exascale Computing Project.