In a nutshell, the issues of nanomaterial characterization revolve around dealing with the difficulty of measuring what one cannot see. Nanomaterials have unique properties on account of their high surface area to mass ratio and their small size. Thus, a wide range of available and state-of-the-art techniques are needed to characterize such materials in order that product performance predictions (including risk assessment and product stewardship when needed) can be made reliably and effectively. Such characterization techniques revolve around the main controlling properties of nanomaterials that can be recalled by means of the 4 S acronym mnemonic:• Size: Properties are seen only in certain size ranges: for example, the wavelength emitted by quantum dots is directly related to the size of the “particle.”• Shape: Carbon nanotubes provide unique properties for materials of extremely high aspect ratio.• Surface: Composition, charge, chemistry, and corona, noting that the surface properties are likely to be very different from the bulk properties.• Solubility: Increased solubility and solubility rate arise through small sizes and increased surface area. Many poorly soluble biopharmaceutical products need to be produced in nanoform to overcome such issues.This chapter explores the main tools available to the researcher in order to gain an insight into the performance prediction and toxicology understanding, including the ultimate fate of nanomaterials in the environment, when the nanomaterials come to be used and employed from a practical perspective.