This book constitutes the refereed proceedings of the tracks and workshops which complemented the 15th European Conference on Software Architecture, ECSA 2021, held in Växjö, Sweden*, in September 2021.  The 15 full papers presented in this volume were carefully reviewed and selected from 17 submissions. Papers presented were accepted into the following tracks and workshops: Industry Track; DE&I - Diversity, Equity and Inclusion Track; SAEroCon - 8th Workshop on Software Architecture Erosion and Architectural Consistency; MSR4SA - 1st International Workshop on Mining Software Repositories for Software Architecture; SAML – 1st International Workshop on Software Architecture and Machine Learning; CASA - 4th Context-aware, Autonomous and Smart Architectures International Workshop; FAACS - 5th International Workshop on Formal Approaches for Advanced Computing Systems; MDE4SA - 2nd International Workshop on Model-Driven Engineering for Software Architecture; Tools and Demonstrations Track; Tutorial Track. *The conference was held virtually due to the COVID-19 pandemic.