Throughout human history, we have long encountered the combination of promise, risk, and uncertainty that accompanies emerging technologies. Nanotechnology is a recent example of an emerging technology that promises to drastically improve existing products as well as allow for creative development of new goods and services. This new technology also has its potential downsides. Industry, academia, and regulatory agencies are all working overtime to assess risks accurately while keeping up with the pace of development. Subtle changes in the physicochemical properties of engineered nanomaterials (ENMs) can influence their toxicity and behavior in the environment and so can be used to help control potential ENM risks. This book attempts to encompass the state of the science regarding physicochemical characterization of ENMs. It illuminates the effort to understand these properties and how they may be used to ensure safe ENM deployment in existing or future materials and products.