The role of dangling bonds in tetrahedrally coordinated systems is analyzed in a variety of different physical situations: tricoordinated silicon atoms at the Si—SiO2interface or in a-Si, vacancies in covalent materials or in compounds, pinning of some impurity levels, dangling bonds at surfaces and interfaces. Emphasis is put on the common features that exist in all these apparently very different situations, resulting in a fairly simple and general understanding of the related physical properties.