As envisioned by Weiser, computing is in the process of being everywhere and becoming invisible. As Milner noticed, the question now is whether we shall understand this ubiquitous computer we are building. This is especially true as designers are more and more using complex techniques for every component of the system and building systems which are made of increasingly heterogenous parts. With this extended editorial, we embark on an exploration journey into the exciting new area of "reliable intelligent environments" (RIEs). Taking the perspective of an RIE engineer, we present a selection of approaches that have been put forward to design, verify, and operate IEs in a manner so that users can rely on Intelligent Environment systems. We outline crucial challenges: the situatedness which exposes IE to challenges similar to those known from robotics and control systems, the embedding of human users and the safety, privacy, and usability requirements thus entailed, and the amounts of data produced by sensors and actuators, which require advanced reasoning and learning mechanisms to handle them in a reliable way in real-time. We also sketch the opportunities reliable IEs provide to developing new markets and products.