Owing to the fact that a Living Reviews in Relativity article may evolve over time, we recommend to cite the article as follows:
Volker Perlick,
"Gravitational Lensing from a Spacetime Perspective",
Living Rev. Relativity 7, (2004), 9. URL (accessed <date>):
http://www.livingreviews.org/lrr-2004-9
The date given as <date> then uniquely identifies the version of the article you are referring to.