Rubber is a typical viscoelastic material and is very easy to handle, as two different properties can be adjusted for elasticity (energy storage) and viscosity (energy loss) according to purpose. Rheology treats this viscoelastic property academically. The mechanical properties of solids such as crosslinked rubber are quantified by viscoelastic measurements. Viscoelastic behavior indicates mechanical properties and indicates the effects of molecular motion and molecular structure. This measuring device is called a rheometer. This section describes the measurement method using a rheometer and examples of evaluation of various rubber materials.