In addition to needing to be hygienic and biocompatible, the base of a complete denture (CD) should provide intimate adaptation to the mucosa of a ridge. The ridge form may differ under function than when at rest because of the compressibility of mucosa. This viscoelastic behavior of ridge mucosa has been a focus of research and a clinical challenge confronted by dentists when making impressions. This article discusses the basic principles of nonpressure mucostatic and selective-pressure functional impression concepts and provides a theoretical and scientific basis for the design of a CD base.