The 'lme4' package provides functions for fitting and analyzing linear, generalized linear, and nonlinear mixed-effects models. It uses S4 classes and methods for model representation and the 'Eigen' C++ library for efficient numerical linear algebra. The package includes functions for model fitting, inference, and diagnostics, with support for various optimization methods and convergence checks. It is designed to be more modular and efficient than the 'nlme' package, offering built-in facilities for likelihood profiling and parametric bootstrapping. The package includes a wide range of functions for model comparison, confidence interval estimation, and diagnostic checks. It also provides data sets for demonstration and examples, along with utilities for handling mixed-effects models. The package is maintained by Ben Bolker and is available on CRAN.The 'lme4' package provides functions for fitting and analyzing linear, generalized linear, and nonlinear mixed-effects models. It uses S4 classes and methods for model representation and the 'Eigen' C++ library for efficient numerical linear algebra. The package includes functions for model fitting, inference, and diagnostics, with support for various optimization methods and convergence checks. It is designed to be more modular and efficient than the 'nlme' package, offering built-in facilities for likelihood profiling and parametric bootstrapping. The package includes a wide range of functions for model comparison, confidence interval estimation, and diagnostic checks. It also provides data sets for demonstration and examples, along with utilities for handling mixed-effects models. The package is maintained by Ben Bolker and is available on CRAN.