Instability is one of the pathologic conditions of the spine for which surgical treatment is increasingly indicated, even if to-date the definition of instability is controversial. Some aspects of the biomechanics of the spine with particular reference to the viscoelastic characteristics of its elements and the involved loads, are reported. The elasticity of the spine is analyzed with an in-depth consideration of the concept of stability and instability from the biomechanical, clinical and radiographic viewpoint.