The industrial sector is being impacted by the Internet of Things due to the increasing availability of computing and communication capabilities in a wide range of devices and equipment deployed in the shop floor. This requires substantial architectural and functional improvements in order to actually manage and exploit the potential of such smart components. The EU FP7 IoT@Work project aims at addressing the issues of Plug&Work configuration and self-management features in automation systems. This paper focuses on presenting the architecture and the main features of the IoT@Work Event Notification Service (ENS) middleware that is used to support an event-driven communication paradigm among devices and services, both inside and outside the factory plant.