Today's multi-device environment, where one user has several network-capable devices, is making the idea of “device-shift” service a reality. In a device-shift service, a user uses multiple cooperating devices to receive a service or a set of cooperating services so that he/she can have a rich and consistent service experience. We are developing a platform architecture to enable such device-shift services for third-party service providers on networks including IP Multimedia Subsystem (IMS) networks and the Internet. We propose a conceptual device-shift model to clarify the concept of device shift and the coverage. Based on this model, we propose a platform architecture for device-shift services and evaluate it through a service use case.