The book "Machine Learning and Knowledge Discovery in Databases. Applied Data Science Track" is a comprehensive collection of research papers presented at the European Conference on Machine Learning and Knowledge Discovery in Databases (ECML PKDD 2021). This conference took place online from September 13 to 17, 2021, due to the COVID-19 pandemic. The volumes contain 210 carefully selected and peer-reviewed contributions out of a total of 869 submitted papers. The content is divided into various thematic sections that cover both fundamental and applied aspects of machine learning and data science. In particular, the section on applied data science addresses relevant topics such as anomaly detection, spatio-temporal data, and applications in healthcare and e-commerce. This collection is a valuable resource for professionals and students engaged with the latest developments in data science and machine learning.