Abstract: Understanding the in situ stress state is crucial in many engineering problems and earth science research. The present article presents new insights into the interaction mechanism between the stress state and faults. In situ stresses can be influenced by various factors, one of the most important being the existence of faults. A fault could significantly affect the value and direction of the stress components. Reorientation and magnitude changes in stresses exist adjacent to faults and stress jumps/discontinuities across the fault. By contrast, the change in the stress state may lead to the transformation of faulting type and potential fault reactivation. Qualitative fault reactivation assessment using characteristic parameters under the current stress environment provides a method to assess the slip tendency of faults. The correlation between in situ stresses and fault properties enhances the ability to predict the fault slip tendency via stress measurements, which can be used to further refine the assessment of the fault reactivation risk. In the future, stress measurements at greater depths and long-term continuous real-time stress monitoring near/on key parts of faults will be essential. In addition, much attention needs to be paid to distinguishing the genetic mechanisms of abnormal stress states and the type and scale of stress variations and exploring the mechanisms of pre-faulting anomaly and fault reactivation.