The `lmerTest` package extends the `lmerMod` class from the `lme4` package by providing $p$-values for $F$ and $t$ tests, implementing Satterthwaite's method for degrees of freedom approximation, and offering Type I-III ANOVA tables. It also includes functions for stepwise model building, least squares means, and multiple comparison tests. The package addresses the limitations of the `lme4` package, such as the lack of $p$-values for $F$ and $t$ tests, and provides more accurate and flexible inference tools for linear mixed effects models. The paper discusses the implementation details, compares different approximation methods, and provides examples using real datasets to illustrate the package's functionality.The `lmerTest` package extends the `lmerMod` class from the `lme4` package by providing $p$-values for $F$ and $t$ tests, implementing Satterthwaite's method for degrees of freedom approximation, and offering Type I-III ANOVA tables. It also includes functions for stepwise model building, least squares means, and multiple comparison tests. The package addresses the limitations of the `lme4` package, such as the lack of $p$-values for $F$ and $t$ tests, and provides more accurate and flexible inference tools for linear mixed effects models. The paper discusses the implementation details, compares different approximation methods, and provides examples using real datasets to illustrate the package's functionality.