<p>Selenites and hydroselenites exhibit a wide structural diversity and are considered as functional materials with a broad spectrum of potential physical properties. The complex crystal chemistry of these compounds is predominantly determined by the peculiarities of chemical bonds between SeO₃ triangles and other polyhedra, as well as the stereochemical activity of the lone electron pair, which makes this family similar in structure to sulfates and sulfites, as well as tellurites. The presence of stable modules in the structures of rare-earth selenites allows for the identification of modular series of crystal structures. This fact enables the classification of known structural types as well as the prediction of new ones using the formalism of modular crystal chemistry. This review highlights the breadth of research on the crystal structures of rare-earth selenites and aims to provide new insights for their more detailed analysis.</p>