This book focuses on the study of the interfacial water using molecular dynamics simulation and experimental sum frequency generation spectroscopy. It proposes a new definition of the free OH groups at water-air interface and presents research on the structure and dynamics of these groups. Furthermore, it discusses the exponential decay nature of the orientation distribution of the free O-H groups of interfacial water and ascribes the origin of the down pointing free O-H groups to the presence of capillary waves on the surface. It also describes how, based on this new definition, a maximum surface H-bond density of around 200 K at ice surface was found, as the maximum results from two competing effects. Lastly, the book discusses the absorption of water molecules at the water-TiO2 interface. Providing insights into the combination of molecular dynamics simulation and experimental sum frequency generation spectroscopy, it is a valuable resource for researchers in the field.