This book constitutes the refereed proceedings of the 13th Software Quality Days Conference, SWQD 2021, which was planned to be held in Vienna, Austria, during January 19–21, 2021. Due to the COVID-19 pandemic, the conference was cancelled and will be merged with SWQD 2022. The Software Quality Days (SWQD) conference started in 2009 and has grown to the biggest conference on software quality in Europe with a strong community. The program of the SWQD conference is designed to encompass a stimulating mixture of practical presentations and new research topics in scientific presentations. The guiding conference topic of the SWQD 2021 is “Future Perspectives on Software Engineering Quality”. The 3 full papers and 5 short papers presented in this volume were carefully reviewed and selected from 13 submissions. The volume also contains 2 invited talks and one introductory paper for an interactive session. The contributions were organized in topical sections named: automation in software engineering; quality assurance for AI-based systems; machine learning applications; industry-academia collaboration; and experimentation in software engineering.