It seems reasonable to suppose that educational practices should be informed by philosophical and scientific understandings of the character and operation of mental processes. Clark and Chambers' 1998 'The Extended Mind' is a seminal paper in the philosophy of mind, but has received limited attention by educational researchers. Their Extended Mind Theory (EMT) provocatively claims that the assumption the mind is restricted to the head is unjustified, and that objects in the environment can function as parts of the mind. Clark and Chamber wrote that the human organism can be "linked with an external entity in a two-way interaction, creating a coupled system that can be seen as a cognitive system in its own right". So, EMT challenges the assumption that the demarcation of skin and skull determines the boundaries of cognition, and demands a radical rethink of the nature of learning. This paper introduces and critiques EMT, initially by way of a discussion of embodiment, an intermediate step in the argument towards extension. It highlights inconsistencies in Clark and Chambers' account, but concludes that the concept of a mind that extends into and interacts with its body and environment is both plausible and helpful.