Insoluble excipients are added to polymeric film coating solutions and dispersions to improve the physical appearance of dosage forms, enhance the stability of photolytic drugs, and aid in processing. These insoluble additives, however, may significantly affect the physical, mechanical, adhesive, and drug-release properties of the films. In this review, the theories of the interactions between polymer and insoluble excipient are addressed. This article also discusses the influence of the concentration, particle size, morphology, and surface chemistry of insoluble excipients on the various polymer properties.