lme4 and nlme are R packages used for fitting linear, generalized linear and nonlinear mixed effects models. For general questions about mixed models use [mixed-model] tag.
R packages used for fitting linear, generalized linear and nonlinear mixed-effects models. For further information check Bates (June 25, 2010), Bates et al (2014), and Pinheiro and Bates (2000).
For questions asking about mixed effects models in general please use mixed-model tag.
Bates, D. (June 25, 2010). lme4: Mixed-effects modeling with R. (unpublished)
Bates, D., Mächler, M., Bolker, B., & Walker, S. (2014). Fitting linear mixed-effects models using lme4. arXiv preprint arXiv:1406.5823.
Pinheiro, J.C., and Bates, D.M. (2000). Mixed-Effects Models in S and S-PLUS. Springer.