The book "Linear Mixed-Effects Models Using R" provides a comprehensive introduction to the application of linear mixed models (LMMs) for analyzing correlated data. These models are significant in various disciplines such as biostatistics, public health, psychometrics, educational measurement, and sociology. The author, Andrzej Galecki, with the support of Tomasz Burzykowski, conveys the theoretical foundations and practical applications of LMMs in the R programming language. The content is structured progressively, starting with simpler linear models and advancing to more complex LMMs. The book includes numerous practical examples to illustrate the concepts and introduces novel R tools that facilitate the application of the models. These tools include new classes of variance-covariance structures for random effects, as well as methods for influence diagnostics and power calculation. The combination of theory, software, and practical applications makes this book a valuable companion for researchers and practitioners in various fields.