First row transition metal complexes with d 4 to d 7 electronic configuration exhibit spin-crossover (SCO), which can be induced by external stimulus such as temperature, pressure, light. The low-spin to high-spin transition has been largely studied, but very little is known about the reverse process. Here, we present a theoretical study of thermal and light-induced high-to-low spin crossover in prototypical Fe(II) complexes. The lifetime of the high-spin state in the thermal process is determined using Fermi's golden rule. With this methodology, we have accurately computed the transfer rate of the HS state thermal relaxation at several time scales (from sub-nanosecond to a few seconds) in two different iron complexes. The use of quasi-degenerate perturbation theory (QDPT2) in the analysis of the LS-HS spin-orbit coupling have allowed to identify 3T1 as the main intermediate state coupling LS and HS states. The light-induced process has been studied using wavepacket quantum dynamics along the main vibrational coordinates (one symmetric and two asymmetric Fe-N stretchings). The study suggests that after the initial excitation from 5T2g to 5Eg state, the population is transferred back to a vibrationally hot 5 T 2g from which a small amount of population is transferred to the 1A1g via the intermediate 3 T 1g. Most of the population remains trapped in the HS state at the time scale of the simulation. Spin-crossover (SCO) is a molecular phenomena wherein the spin state of the system changes because of an external stimulus such as pressure, temperature, light irradiation or the influence of a magnetic field. This process commonly takes place from the diamagnetic thermally stable low-spin state to the paramagnetic