Metaphor is one of the most intriguing topics in cognitive linguistics. In this thesis, I propose a theory of metaphor from within a neurocognitve framework, specifically the relational network model of Sydney Lamb. The basis of metaphor in this model is the shared properties of concepts in the network. Metaphoric associations are created through evoked similarity in a given context. There are three aspects of metaphor in the relational network model: creative nonce metaphors, learned metaphoric expressions, and emergent conceptual metaphors. For metaphoric expressions, there is a cline from nonce metaphors to dead metaphors--expressions that are more toward the nonce end of the cline require more input from context for correct comprehension. Studies of subjects with right hemisphere brain damage show that they have problems with choosing the appropriate context and problems comprehending metaphor. These findings support the theory of metaphor presented in this thesis.