The growing trend of multi-device ownerships creates a need and an opportunity to use applications across multiple devices. However, in general, the current app development and usage still remains within the single-device paradigm, falling far short of user expectations. In this talk, I will introduce several approaches towards multi-device mobile platforms, in particular, with a focus on M+, an extension of Android that supports cross-device functionality sharing in a transparent manner (MobiSys ’17). As a platform-level solution, M+ enables unmodified Android applications to utilize not only application functionalities but also system functionalities across devices, as if they were to utilize them inside the same device. Our experimental results show that M+ enables transparent cross-device sharing for various functionalities and achieves performance close to that of within-device sharing unless a large amount of data is transferred.