This self-contained textbook aims to introduce the physics of structurally disordered condensed systems at the level of advanced undergraduate and graduate students. The topics discussed include the geometry and symmetries of the building blocks commonly used to obtain atomic structures, the various kinds of disorder, the phenomenology and the main theories of the glass transition, investigation of the structure of amorphous systems, the dependence of system structure on its dimensions (clusters), and the case of positional order in the absence of translational order (quasicrystals).