The physical energy that we refer to as a word, whether in isolation or embedded in sentences, takes its meaning from the knowledge stored in our brains through a lifetime of experience. Much empirical evidence indicates that, although this knowledge can be used fairly flexibly, it is functionally organized in 'semantic memory' along a number of dimensions, including similarity and association. Here, we review recent findings using an electrophysiological brain component, the N400, that reveal the nature and timing of semantic memory use during language comprehension. These findings show that the organization of semantic memory has an inherent impact on sentence processing. The left hemisphere, in particular, seems to capitalize on the organization of semantic memory to pre-activate the meaning of forthcoming words, even if this strategy fails at times. In addition, these electrophysiological results support a view of memory in which world knowledge is distributed across multiple, plastic-yet-structured, largely modality-specific processing areas, and in which meaning is an emergent, temporally extended process, influenced by experience, context, and the nature of the brain itself.