To cite 'mediation' in publications use:

Tingley D, Yamamoto T, Hirose K, Keele L, Imai K (2014). “mediation: R Package for Causal Mediation Analysis.” Journal of Statistical Software, 59(5), 1–38. http://www.jstatsoft.org/v59/i05/.

For the underlying methods please cite the following papers:

Imai K, Keele L, Yamamoto T (2010). “Identification, Inference, and Sensitivity Analysis for Causal Mediation Effects.” Statistical Science, 25(1), 51–71. http://imai.princeton.edu/research/mediation.html.

Imai K, Keele L, Tingley D (2010). “A General Approach to Causal Mediation Analysis.” Psychological Methods, 15(4), 309–334. http://imai.princeton.edu/research/BaronKenny.html.

Imai K, Keele L, Tingley D, Yamamoto T (2011). “Unpacking the Black Box of Causality: Learning about Causal Mechanisms from Experimental and Observational Studies.” American Political Science Review, 105(4), 765–789. http://imai.princeton.edu/research/mediationP.html.

Imai K, Yamamoto T (2013). “Identification and Sensitivity Analysis for Multiple Causal Mechanisms: Revisiting Evidence from Framing Experiments.” Political Analysis, 21(2), 141–171. http://imai.princeton.edu/research/medsens.html.

Imai K, Keele L, Tingley D, Yamamoto T (2010). “Causal Mediation Analysis Using R.” In Vinod HD (ed.), Advances in Social Science Research Using R. Springer-Verlag, New York.

Corresponding BibTeX entries:

  @Article{,
    title = {{mediation}: {R} Package for Causal Mediation Analysis},
    author = {Dustin Tingley and Teppei Yamamoto and Kentaro Hirose and
      Luke Keele and Kosuke Imai},
    journal = {Journal of Statistical Software},
    year = {2014},
    volume = {59},
    number = {5},
    pages = {1--38},
    url = {http://www.jstatsoft.org/v59/i05/},
  }

  @Article{,
    title = {Identification, Inference, and Sensitivity Analysis for
      Causal Mediation Effects},
    author = {Kosuke Imai and Luke Keele and Teppei Yamamoto},
    journal = {Statistical Science},
    year = {2010},
    volume = {25},
    number = {1},
    pages = {51--71},
    url = {http://imai.princeton.edu/research/mediation.html},
  }

  @Article{,
    title = {A General Approach to Causal Mediation Analysis},
    author = {Kosuke Imai and Luke Keele and Dustin Tingley},
    journal = {Psychological Methods},
    year = {2010},
    volume = {15},
    number = {4},
    pages = {309--334},
    url = {http://imai.princeton.edu/research/BaronKenny.html},
  }

  @Article{,
    title = {Unpacking the Black Box of Causality: Learning about
      Causal Mechanisms from Experimental and Observational Studies},
    author = {Kosuke Imai and Luke Keele and Dustin Tingley and Teppei
      Yamamoto},
    journal = {American Political Science Review},
    year = {2011},
    volume = {105},
    number = {4},
    pages = {765--789},
    url = {http://imai.princeton.edu/research/mediationP.html},
  }

  @Article{,
    title = {Identification and Sensitivity Analysis for Multiple
      Causal Mechanisms: Revisiting Evidence from Framing Experiments},
    author = {Kosuke Imai and Teppei Yamamoto},
    journal = {Political Analysis},
    year = {2013},
    volume = {21},
    number = {2},
    pages = {141--171},
    url = {http://imai.princeton.edu/research/medsens.html},
  }

  @InCollection{,
    title = {Causal Mediation Analysis Using R},
    author = {Kosuke Imai and Luke Keele and Dustin Tingley and Teppei
      Yamamoto},
    booktitle = {Advances in Social Science Research Using R},
    year = {2010},
    editor = {H. D. Vinod},
    publisher = {Springer-Verlag},
    address = {New York},
  }