The emerging fog paradigm has been attracting increasing interest from both academia and industry, due to the low-latency, resilient, and cost-effective services it can provide. Many fog applications, such as video mining and event monitoring, rely on data stream processing and analytics, which are very popular in the cloud, but have not been comprehensively investigated in the context of fog architecture. In this article, we present the general models and architecture of fog data streaming, by analyzing the common properties of several typical applications. We also analyze the design space of fog streaming with the consideration of four essential dimensions (system, data, human, and optimization), where both new design challenges [...]