Age of information (AoI) was introduced in the early 2010s as a notion to characterize the freshness of the knowledge a system has about a process observed remotely. AoI was shown to be a fundamentally novel metric of timeliness, significantly different, to existing ones such as delay and latency. The importance of such a tool is paramount, especially in contexts other than transport of information, since communication takes place also to control, or to compute, or to infer, and not just to reproduce messages of a source. This volume comes to present and discuss the first body of works on AoI and discuss future directions that could yield more challenging and interesting research.1 In this volume we take into consideration works that have been published no later than June 2017. freshness of such information is paramount in a wide range of information, communication, and control systems. By now, age has been studied with considerable diversity of systems, being as a concept, a performance metric, and a tool.The purpose of this volume is to present a critical summary of this first body of works performed on AoI and discuss future research directions. Already at this early point we need to put down our first disclaimer: we have chosen to treat the early works with significantly more detail, going deeper in the derivations and presenting more results and insights from them than we do with more recent works. The reason for this is to achieve a tutorial nature in the volume, which can provide a solid ground of the AoI as a concept. Moreover, the first works, which we chose to present in more detail than the rest, aim to provide fundamentally new knowledge in the premise of maintaining information fresh in a system. This basic goal opens up a wide range of communication contexts that span from estimation and prediction, to applications such as vehicular networks and information caching, to name a few.With this in mind, we begin this volume presenting the AoI concept as it was originally introduced. For this, we discuss the original models of Kaul, Gruteser and Yates of [33], considering a system where a source is transmitting packets containing status updates to a destination. The analysis presented is based on a simple queueing model. Already in that work the minimization of AoI was shown to be non-trivial for the source sampling methods studied. However, it had already become clear that timely updating a destination about a remote system is neither the same as maximizing the utilization of the communication system, nor of ensuring that generated status updates are received with minimum delay. This is because utilization can be maximized by making the source send updates as fast as possible which would lead to the destination receiving delayed statuses because messages are backlogged in the communication system studied. In this case, delay suffered by the stream of status updates can be reduced by decreasing the rate of updates. Alternatively, decreasing the update rate can also lead to the destination having unnecessa...