Why do languages have the words they have, and not some other set of words? While certainly there is some arbitrariness in the lexicon (English ‘frog’ vs. Spanish ‘rana’), there is just as surely some systematicity or functionality in it as well. What exactly might the nature of this systematicity or functionality be? For example, might the lexicon be efficiently adapted for communication, learning, memory storage, retrieval, or other cognitive functions? This paper critically reviews evidence that natural language lexicons efficiently carve up semantic fields (e.g., color, space, kinship) and have phonological forms that are similarly efficient when the aggregate lexicon is considered. The paper also suggests additional ways functionalism in lexicons might be assessed, and speculates on how functional lexicons may have arisen.