This book focuses on defining the principles of bioadhesive delivery systems based on hydrogels to biological surfaces that are covered by mucus. An overview of the last decade's discoveries on mucoadhesion and applications of mucoadhesive hydrogels as drug carriers is given. Techniques that are frequently used to study the adhesion forces and physicochemical linteractions between hydrogel, mucus, and the underlying mucosa are reviewed. Typical examples of applications of mucoadhesive hydrogels to mucosal routes of delivery are given. Finally, the perspectives of the application of these polymers in drug delivery are discussed.