This book constitutes the refereed proceedings of the 12th European Conference on Software Architecture, ECSA 2018, held in Madrid, Spain, in September 2018. The 17 full papers presented together with 7 short papers were carefully reviewed and selected from 96 submissions. They are organized in topical sections as follows: Self-Adaptive Architectures, IoT Architectures, Embedded and Cyber-Physical Systems, Microservices Architectures, Service-Oriented Architectures, Architectural Design Decisions, Software Architecture in Practice.