Symmetry is introduced as a basic notion of physics and, in particular, for soil mechanics also. Isotropy and anisotropy are discussed. A special case of isotropy of space is the principle of material frame indifference which plays an eminent role in the development of constitutive equations. The geometric scaling is discussed together with the notion of a simple material, which is – often unconsciously – basic in geotechnical engineering. Invariance with respect to stress and time scales is discussed. Mechanical similarity and the associated Pi theorem is shown to be the basis for the evaluation of so-called physical simulations with model tests.