Owing to the fact that a Living Reviews in Relativity article may evolve over time, we recommend to cite the article as follows:
Jeffrey Winicour,
"Characteristic Evolution and Matching",
Living Rev. Relativity 12, (2009), 3. URL (accessed <date>):
http://www.livingreviews.org/lrr-2009-3
The date given as <date> then uniquely identifies the version of the article you are referring to.