Relativistic outflows are ubiquitous in high-energy cosmic phenomena. Whether their reservoir of free energy at launch is in the form of bulk kinetic, magnetic or gravitational, it is ultimately channeled into energetic particles and non-thermal radiation. A key question is to understand how this transfer operates efficiently and under which conditions. Here, I review some of the recent developments in the modeling of two particle acceleration processes, namely relativistic diffusive shock acceleration and relativistic magnetic reconnection from the perspective of ab-initio particle-in-cell simulations. These results and their astrophysical implications are briefly discussed in the context of relativistic outflows.