Abstract: Shock waves are ubiquitous astrophysical phenomena associated with powerful sources, e.g., supernovae, active galactic nuclei, and gamma-ray bursts. These shocks are collisionless, meaning that the energy is transferred between particles, electromagnetic fields, and bulk flows by wave-particle interactions, rather than binary Coulomb collisions. Such shocks are known to be efficient accelerators of charged particles and are plausible sites of cosmic ray production. However, the primary acceleration mechanism, diffusive shock acceleration, does not describe physics on particle kinetic scales. Since shock acceleration is in general a nonlinear and multiscale process, particle-scale phenomena are crucial. I will discuss the shock microphysics and particle acceleration mechanisms revealed by fully kinetic numerical simulations.