mlr3 is an award-winning ecosystem of R packages that have been developed to enable state-of-the-art Machine Learning capabilities in R. Applied Machine Learning Using mlr3 in R gives an overview of flexible and robust Machine Learning methods, with an emphasis on how to implement them using mlr3 in R. It covers various key topics, including basic Machine Learning tasks, such as building and evaluating a predictive model; hyperparameter tuning of Machine Learning approaches to obtain peak performance; building Machine Learning pipelines that perform complex operations such as pre-processing followed by modelling followed by aggregation of predictions; and extending the mlr3 ecosystem with custom learners, measures, or pipeline components. The book is primarily aimed at researchers, practitioners, and graduate students who use Machine Learning or who are interested in using it. It can be used as a textbook for an introductory or advanced Machine Learning class that uses R, as a reference for people who work with Machine Learning methods, and in industry for exploratory experiments in Machine Learning.