Cavity liners have traditionally been used in direct and indirect restorations for purposes such as promoting reparative dentin and neutralizing acids. Today, liners are used when resin composites are used as the restorative materials on the posterior teeth, but for a different reason. Clinically, liners are considered to decrease sensitivity and wet the cavity better than restorative composites because of their flowability, adaptation to the dentinal surface, and adhesion. As bonding systems and composite materials continue to improve and become better understood, so do the techniques for placing composites. This has led to the reassessment of the clinical relevance and function of liners. Some clinicians don't use cavity liners, assuming they are a thing of the past; some use composite liners, and others use resin-modified glass ionomer liners. Additionally, there is not a clear agreement over the function of liners, such as when and why they should be used or what type of liner material would provide the best performance for a particular clinical situation. This article attempts to clarify some of the confusion surrounding the use of liners by reviewing the available literature on the subject and attempting to give evidence-based rationale for the use and protocol for the clinician.