Abstract: Quantum fields living on cosmological spacetimes can experience particle production due to their interaction with the evolving background. This effect is particularly relevant for models of the very early Universe, when the energy density generated through this process may back-react on the cosmological expansion. I will review the basics of cosmological quantum particle production and outline a numerical approach capable of solving the back-reaction problem in regimes dominated by this effect. As an illustration, I will present the back-reaction calculation for a quantized scalar field on a cosmological bounce scenario. Finally, I will discuss the relevance of quantum particle production/annihilation to models of the early Universe.