Nanocomposite hydrogel is defined as a reinforced hydrophilic three-dimensional network, consisting of at least a crosslinked polymeric matrix and a nanoscale reinforcing agent. They have unique superior properties and tailored functionalities. Nanocomposite hydrogels, as newly born multi-functional materials in the 21st century, have attracted much scientific attention and they are believed to be a revolutionary type of material. The objective of this book is to introduce the fundamentals of nanocomposite hydrogels and review their emerging applications in different practical fields.