In 50 chapters on different topics and country experiences, leading international experts explore how issues and developments such as social media and AI have impacted the content, quality and organization of policy advice for modern governments. They discuss how the nature and deployment of policy expertise is changing amidst the fragmentation of existing information ecosystems and growing distrust in traditional actors and institutions. The Handbook analyses the features and problems of existing studies and practices such as evidence-based policy-making and addresses the future of policy advising, illustrating the impact and implications of ongoing shifts towards more pluralistic and social-media-driven sources of policy knowledge.