Low-Energy Excitations in Disordered Solids: A Story of the 'Universal' Phenomena of Structural Tunneling : World Scientific Publishing : 9789811217241 : 9811217246 : 16 May 2021 : The subject of low-energy excitations in glasses has evolved, since they were first noticed 60 years ago in the thermal properties of oxide glasses and polymers, into a multi-focused amalgam that includes a wide range of other materials containing disorder: amorphous semiconductors and metals, poly-crystalline metals, doped-, mixed- and quasi-crystals, surface adsorbates, . . . and topics such as dephasing of quantum states and interferometer noise. The structures causing these effects are known in a few cases, but the central mystery remains: what drives their existence and their uniform properties. A fairly simple empirical description serves well to describe but not to explain them in terms of the material's formation dynamics.