Owing to the fact that a Living Reviews in Relativity article may evolve over time, we recommend to cite the article as follows:
Antonio De Felice and Shinji Tsujikawa,
"f(R) Theories",
Living Rev. Relativity 13, (2010), 3. URL (accessed <date>):
http://www.livingreviews.org/lrr-2010-3
The date given as <date> then uniquely identifies the version of the article you are referring to.