Korean speech rhythms differ in interesting ways from those of English, especially in the role of intonationally defined prosodic groupings, which influence the pronunciation of consonants and vowels as profoundly as does stress in English. This account of Korean intonational rhythms is based on experiments that suggest a hierarchy of intonationally defined groupings, which exert different influences on the consonants and vowels at their edges. For example, the smaller accentual phrase affects the pronunciation of a class of consonants which are voiceless (sounding like Spanish "p" or "ch") in phrase-initial position, but become voiced (like English "b" or "j") in phrase-medial position. The larger intonational phrase also affects these consonants by making exceptions to the generalization that they will be pronounced as nasals (like French "m" or "gn") when they occur before another nasal. Other experiments show that speakers vary the intonational groupings that they assign to any string of words, in ways that reflect influence from many other aspects of the utterance including overall speech tempo, the words' syntactic structure and relative predictability, and the signalling of narrow focus of attention on any particular word. Significantly, the influence of focus of attention was paramount, contrary to many current linguistic theories which propose syntactic structure as the primary determinant of prosodic rhythms.