Nothing expresses the vitality, history, and character of a language quite like its idioms. Ask any non-native English speaker to make sense of a phrase such as "beg the question" or "keep your eyes peeled"; they can't do it. Idioms don't translate, which is what makes them such intriguing mirrors of how a culture evolves along with its speech. The American Heritage Dictionary of Idioms includes almost 10,000 of these figures of speech, slang phrases, clichés, colloquialisms, and proverbs, from "ace in the hole" to "zoom in on." Each entry defines an idiom, uses it in a sentence, then pinpoints its historical origins when possible. Some idioms, it turns out, preserve words or word uses that have otherwise fallen out of use ("one fell swoop"); others allude to long-forgotten catch phrases from movies or advertising ("more bounce for the ounce"). Consider, for instance the phrase "funny bone"--actually a pun on "humerus," the Latin name for the bone of the upper arm. Or the expression "moment of truth," a translation from the Spanish phrase originally referring to bullfighting--and first popularized, not surprisingly, by Ernest Hemingway. The American Heritage Dictionary of Idioms is like an archeological dig through the vernacular, and it unearths treasures such as these on every one of its pages. All those interested in language or its history should keep a copy on their library shelves.