As power systems around the world transform, power system flexibility has become a global priority. A range of operational, policy and investment-based interventions are available to render modern systems more flexible, thereby facilitating cleaner, and more reliable, more resilient, and more affordable energy. This report identifies challenges and opportunities to unlock system flexibility and accelerate power system transformation (PST) efforts. It provides an overview of the policy, regulatory and market instruments which can be implemented in different power sector contexts to mitigate these challenges. Importantly, all power system assets, including variable renewable energy, can provide flexibility services, if enabled by proper policy, market and regulatory frameworks. These assets include power plants, electricity networks, energy storage and distributed energy resources. A wealth of known strategies, approaches and instruments can be readily applied and adapted to power systems. These include modifications to: energy strategies; legal frameworks; policies and programmes; regulatory frameworks; market rules; system operation protocols; and connection codes. Moving forward, updating system flexibility policies to match the pace of technological development can help to accelerate global PST, while ensuring that all classes of power system assets are able to receive fair remuneration for the flexibility services they are capable of providing.