How do our neighbours do product line engineering?

Product line engineering (PLE) approaches have been followed in industry for hardware and software solutions for more than three decades now. However, the different engineering disciplines (e.g. mechanics, electrics, software) have developed and evolved their approaches within their own realms, which is fine as long as there is no need for integrated approaches. Driven by the increasing complexity of systems, there is a rising need for interdisciplinary systems engineering these days. Companies engineering cyber-physical systems and their components have to integrate product line engineering approaches across the involved engineering disciplines to enable a global optimization of portfolio, solution structures, and assets along their lifecycle. From a bird's-eye view, there is noticeable commonality but also variety in the approaches followed for PLE in the different engineering disciplines, which renders the integration of approaches a non-trivial endeavour. In order to foster the development of integrated PLE approaches, this paper explores, maps, and compares PLE approaches in the field of hardware and software engineering. Furthermore, the paper identifies integration opportunities and challenges. As the paper targets industrial practitioners, it mainly provides references to respective industrial events and material and does not fully cover related work in the respective research communities.