Historical paintings are complex multilayered and dynamic systems whose stability is strongly influenced by environmental conditions. In recent years, growing attention has been directed towards the impact of water exposure on oil paints, as a consequence of dramatic events, uncontrolled humidity levels or interventions during conservation treatments. This review provides a critical overview of recent research on this topic, gathering studies mainly carried out on laboratory mock-ups subjected to controlled exposures such as RH cycling, immersion, or surface contact. Factors such as pigment composition, polymerization state, and stratigraphy govern the extent of swelling, diffusion, and chemical changes upon moisture ingress. An emerging body of work indicates that water in paint films does not behave as a single uniform phase, but is distributed among distinct populations including tightly bound water, nanometric clusters, and more mobile fractions in micro- or macroporosities. These different populations play a key role in water transport and reactivity in the paint matrix. We discuss the underlying mechanisms of water uptake and transport, and their consequences for the chemical and mechanical integrity of paint films. By consolidating recent advances, this review provides a foundation for further research in this field, highlighting current gaps and opening new avenues of investigation.