"Second Life" (SL) is currently the most mature and popular multi-user virtual world platform being used in education. Through an in-depth examination of SL, this article explores its potential and the barriers that multi-user virtual environments present to educators wanting to use immersive 3-D spaces in their teaching. The context is set by tracing the history of virtual worlds back to early multi-user online computer gaming environments and describing the current trends in the development of 3-D immersive spaces. A typology for virtual worlds is developed and the key features that have made unstructured 3-D spaces so attractive to educators are described. The popularity in use of SL is examined through three critical components of the virtual environment experience: technical, immersive and social. From here, the paper discusses the affordances that SL offers for educational activities and the types of teaching approaches that are being explored by institutions. The work concludes with a critical analysis of the barriers to successful implementation of SL as an educational tool and maps a number of developments that are underway to address these issues across virtual worlds more broadly.