While students do learn real-world knowledge and skills in virtual worlds, educators have yet to adequately theorise how students' virtual world experiences bring about this learning. This paper critically reviewed theories currently used to underpin empirical work in virtual worlds for education. In particular, it evaluated how applicable these theories' learning mechanisms are to virtual world-based learning. Eleven theories were identified from 80 journal papers. Four learning mechanisms were found to be applicable to virtual worlds: that students learn through reflection, verbal interactions, mental operations, and vicarious experiences. However, one commonly implied learning mechanism was found to be inapplicable to virtual worlds: that students undergo a physical sensorimotor experience of the real-world phenomenon through their virtual world actions. An alternative theory is needed to explain how students' virtual world actions might bring about learning, so as to help educators determine what exactly students can learn by performing virtual world actions.