Release engineering deals with all activities in between regular development and delivery of a software product to the end user, i.e., integration, build, test execution, packaging and delivery of software. Although research on this topic goes back for decades, the increasing heterogeneity and variability of software products along with the recent trend to reduce the release cycle to days or even hours starts to question some of the common beliefs and practices of the field. The RELENG workshop series aims to provide a highly interactive forum for researchers and practitioners in release engineering to: (1) make researchers aware of the challenges and research opportunities for modern release engineering, and practitioners of the latest research results; (2) share experiences with practical approaches, tools, methods and techniques that are enabling rapid, robust deployment, and (3) build and maintain connections between the different communities. The RELENG 2015 workshop will consist of a keynote, practitioner talks, paper presentations, working groups and a fishbowl panel for semi-structured group discussions. Two keynotes, presented by industrial release engineers, will set the stage for the workshop, introducing the challenges of modern software companies related to release engineering. RELENG 2015 will also feature a poster session to allow researchers and practitioners to present, show-case, and discuss their most recent advances, experiences, and challenges in release engineering in an informal setting. Since bringing together practitioners and researchers is the core goal of RELENG, one of the co-organizers is a release engineer at Mozilla and half of the PC consists of release engineers, so we guarantee that each paper or abstract submission receives at least one review from a practitioner.