Low and time-varying inertia resulting from the increasing penetration of non-synchronous generating units leads to quick inertia responses after power imbalances. The complex nature of the modern network, with so many unclear scenarios, suggests that this is the time for challenges and changes. This triggered my interest in working in this area to develop solutions for new challenges facing the modern power system. Therefore, I have proposed offline and online inertia estimation methods with reduced computation burdens. I have also proposed a long-range inertia forecasting technique to identify when the network is likely at risk in a reasonable time ahead for planning, controlling, and protecting the network with reduced inertia. Furthermore, I have proposed inertia optimization to minimize the operation cost of modern and future networks. My research contributions are intended to assist network operators in applying suitable actions to alleviate stability issues and mitigate the effects of fast-frequency responses. Future research includes coordination between synthetic and conventional synchronous generators inertias and the effects of weather forecasts on inertia prediction.