Owing to the fact that a Living Reviews in Relativity article may evolve over time, we recommend to cite the article as follows:
Kostas D. Kokkotas and Bernd Schmidt,
"Quasi-Normal Modes of Stars and Black Holes",
Living Rev. Relativity 2, (1999), 2. URL (accessed <date>):
The date given as <date> then uniquely identifies the version of the article you are referring to.