**1. Introduction**

Almost all of biological tissues are viscoelastic and their viscoelastic mechanical properties are important in their characteristic functions. This is because constituents of tissues cells, extracellular matrices, structural proteins, and so on are viscoelastic. Even hard tissues have been shown to be viscoelastic. For long, main sample material for investigating the viscoelasticity has been amorphous polymeric materials, which brought about the remarkable development in phenomenological theories. Constructing mechanistic images, on the other hand, of the viscoelasticity of polymeric materials had been difficult because of dearth of the materials structural information. Contrary to these materials, for biological tissues, many structural investigations have been made and as a result detailed structural information is available. Making use of the information, it will be possible to investigate the viscoelastic properties of biological materials on the basis of their structure. Such a study will contribute to the construction of molecular theory for the viscoelasticity in amorphous materials.

In this chapter, some examples of viscoelastic nature of biological materials and then their relevance to the structure would be presented. In some cases, a mechanistic model for the viscoelasticity will be presented. As measuring method varies depending on the modulus value of the specimens, the various methods used in studying viscoelastic properties of biological materials will be illustrated.
