Owing to the fact that a Living Reviews in Relativity article may evolve over time, we recommend to cite the article as follows:
Aled Jones and Anthony N. Lasenby,
"The Cosmic Microwave Background",
Living Rev. Relativity 1, (1998), 11. URL (accessed <date>):
The date given as <date> then uniquely identifies the version of the article you are referring to.