In this paper, we examine the state of the art in augmented reality (AR) for mobile learning. Previous work in the field of mobile learning has included AR as a component of a wider toolkit for mobile learning but, to date, little has been done that discusses the phenomenon in detail or that examines its potential for learning, in a balanced fashion that identifies both positive and negative aspects of AR. We seek to provide a working definition of AR and examine how it is embedded within situated learning in outdoor settings. We also attempt to classify AR according to several key aspects (device/technology; mode of interaction; type of media involved; personal or shared experiences; if the experience is portable or static; and the learning activities/outcomes). We discuss the technical and pedagogical challenges presented by AR before looking at ways in which AR can be used for learning. Lastly, the paper looks ahead to what AR technologies may be on the horizon in the near future.