This chapter discusses the important physicochemical properties of composition, size, shape, surface properties, and aggregation/agglomeration/dispersion state for nanomaterial characterization. The emission properties of luminescent nanomaterials may yield information on sample agglomeration or aggregation, although typically this will require a detailed understanding of the photophysics of the material and possible energy transfer or quenching mechanisms. One of the most difficult things to address with respect to the state of aggregation, agglomeration, or dispersion of a nanomaterial is the necessity of assessing the dispersion state in an environment relevant to its end use. Beyond the role of surface charge on the colloidal stability of nanomaterial suspensions, the surface functional groups have a major impact on the adsorption of species from the environment and can also modulate the hydrophobicity/hydrophilicity and, in some cases, size and morphology of the material.