The field of Virtual Reality continues to evolve to provide an ever-greater sense of immersion to the user. However, VR experiences are still primarily constrained through the human senses of vision and audition, with some interest in haptic (mainly vibrotactile) applications. Only recently have olfactory displays- technologies that generate and deliver scent stimuli- been examined to provide the sense of smell to the human olfactory organ in virtual environments. This paper presents a classification and review of olfactory-enhanced virtual reality systems, particularly those that deployed a Head-Mounted Display (HMD) or Cave Automatic Virtual Environment (CAVE) system. In addition, the paper provides a discussion of the various technological and design challenges for developing an olfactory display suitable for enhancing virtual reality experiences. Finally, the paper proposes future perspectives on the field and includes a table summarizing the characteristics and features of the reviewed systems.