Despite advances in the long-range electrostatic double-layer force, which depends strongly on ionic strength in water by using theoretical models such as DLVO (Derjaguin, Landau, Verwey, and Overbeek), the structure of confined water in air still remains widely unknown and has led to a variety of unexplained phenomena. This book bridges that gap by introducing a newly developed scanning probe miscroscopy (SPM) approach, which enables one to probe confined water at the molecular and atomic scale. Written by the developer of SPM, this book covers this new approach, as well as original approaches to addressing general interfacial water issues. It also introduces the cantilever-based optical interfacial force microscope (COIFM), which was invented by the author along with the methodology. The improved understanding will contribute to liquid-based nano- and bio-technologies such as lab-on-a-chip technologies, nanofluidic devices, dip-pen nanolithography, nano-oxidation, water-based granular interactions, liquid-based nanolubricants, hydration layers in biopolymers, manipulation of biomolecules, protein folding, stability of colloid suspensions, enzyme activity, swelling in clays, development of bioactive surfaces, water columns and ion channeling in membranes and scanning probe microscopy (SPM). It will also contribute to the improved performance of moving components in silicon-based micro-electro-mechanical system (MEMS) devices, where water plays a key role in interfacial interactions.