This book offers a timely and comprehensive exploration of how generative AI technologies are reshaping the landscape of online disinformation, dramatically lowering the barriers to creating realistic synthetic text, imagery, and audio at scale. While online disinformation has long challenged societies and individuals, the acceleration of generative AI has intensified these risks, making traditional detection and protection measures increasingly inadequate. In response, researchers are developing advanced AI-driven tools and methods to detect, mitigate, and study the rapidly evolving tactics of disinformation campaigns, with emphasis on algorithmic detection, media verification, and the broader implications for information integrity.