Owing to the fact that a Living Reviews in Relativity article may evolve over time, we recommend to cite the article as follows:
Gregory B. Cook,
"Initial Data for Numerical Relativity",
Living Rev. Relativity 3, (2000), 5. URL (accessed <date>):
http://www.livingreviews.org/lrr-2000-5
The date given as <date> then uniquely identifies the version of the article you are referring to.