A review is given of the relation between pairing, quasi-spin algebras and seniority. The former two concepts are closely connected, the relation being that the quasi-spin formalism allows an efficient solution of the pairing problem. Seniority is of much wider applicability than either pairing or quasi-spin and applications can even be extended to encompass the notion of partial symmetry. A recent application of partial conservation of seniority and its importance for nuclear isomers is discussed.