The Robot Operating System (ROS) is the de-facto standard for robotic software. If on one hand ROS is helping roboticists, e.g., by providing a standardized communication platform, on the other hand ROS-based systems are getting larger and more complex and could benefit from good software architecture practices. This paper presents an observational study aimed at (i) unveiling the state-ofthe- practice for architecting ROS-based systems and (ii) providing guidance to roboticists about how to properly architect ROS-based systems. To achieve these goals, we (i) build a dataset of 335 GitHub repositories containing real open-source ROS-based systems, (ii) mine the repositories for extracting the state of the practice about how roboticists are ...