Scalar condensates are very common objects in cosmology. For example, the inflaton field can be viewed as a scalar condensate before it completely dissipates into ordinary particles during reheating. Axion condensates may have been formed through the vacuum-misalignment mechanism. In this talk, I will discuss the dissipation of oscillating homogeneous scalar backgrounds in flat spacetime and an expanding universe using nonequilibrium quantum field theory. The latter naturally captures the thermal effects and backreaction effects. For quasi-harmonic oscillations, we adopt the multi-scale analysis to obtain analytical approximate expressions for the self-consistent evolution of the scalar condensates in terms of the retarded self-energy and retarded proper four-vertex function, whose imaginary parts characterize different condensate decay channels. At finite temperatures, there are many new condensate decay channels that would be absent at zero temperature. These new channels could play an important role in ensuring a complete dissipation in an expanding universe. The talk is based on the following two papers: JHEP 11 (2021) 160 [arXiv:2108.00254 [hep-ph]]; JHEP 11 (2022) 075 [arXiv:2202.08218 [hep-ph]]