The aim of the Master thesis was to deposit wüstite (Fe_1-xO) thin films by reactive magnetron sputtering. Wüstite has a wide range of applications and is an important phase for many processes in the future hydrogen economy, including photocatalyst for hydrogen production by water splitting or CO₂-free hydrogen-based reduction of iron ore. Sputtering as a non-equilibrium process for wüstite thin-film deposition is hugely advantageous as wüstite only exists as a stable phase at high temperatures, and sputtering can access these metastable phases. Despite this, systematic studies reporting on wüstite thin-film deposition by reactive magnetron sputtering, and descriptions of the growth mechanisms, are not yet available. This is addressed here by depositing wüstite onto Si (001) substrates, and systematically varying the deposition conditions such as oxygen flux, substrate-target distance, and substrate temperature. This thesis has two main outcomes: Firstly, it was found that surface diffusion-controlled growth dominates for the formation of the metastable wüstite phase at room temperature. Thereby the diffusion is influenced by the substrate temperature where non-intentional heating yielded the largest wüstite phase fraction. The other regulating parameter for the phase formation of the film is the target-substrate distance. This distance determines the energy of the arriving particles, and samples containing the largest volume fraction of wüstite had the shortest target-substrate distances. The second crucial factor is the optimal ratio of deposition rate and oxygen supply rate. This can be controlled either by the target-substrate distance which influences the deposition rate or the oxygen flux which regulates the oxygen supply rate. Overall, this work provides novel insights into the reactive sputtering process of wüstite thin films and how the oxygen flux, the target-substrate distance and the substrate temperature influence the process, leading to improved control over the film growth process of wüstite.