It is rare to have a pharmaceutical dosage form presented with just the pure active pharmaceutical ingredient because the drug substance does not possess adequately desirable physical attributes to be processed into the final dosage form. Consequently, additives or excipients which are inert ingredients serving a functional purpose are added to enhance the overall properties of the final formulation for ease of processability or drug product performance. Variability in excipients arises from source of raw materials and in synthesis/manufacturing process resulting in different mechanisms of action, optimum concentration for use and final product performance including drug-excipient interactions. Unsurprisingly, variability of excipients has been well researched within specific focus areas. This review article aims to look at how different pharmaceutical processes are influenced by the differences in excipient properties as well as advanced analytical and statistical modeling techniques used in their development and characterization.