The unstressed high vowels /i/ and /u/ are subject to extreme shortening, devoicing, or elision in certain environments in standard modern Greek. This process goes on below the consciousness of most native speakers in their own speech; however, they will recognize it in speakers of northern Greek dialects where unstressed /i/ and /u/ are frequently elided (see Newton, 1972; Papadopoulos, 1926). In the phonology of the language, vowel reduction and elision are treated as a collection of optional ‘fast speech’ rules (e.g. Theophanopoulou-Kontou, 1973). In this study, they are considered as related stages in the same phonetic process, which will be described from both an auditory and acoustic point of view. An analysis of read texts and spontaneous speech samples by a number of educated speakers from Athens and Thessalonika shows that vowel reduction is not purely optional, but depends on a number of factors, the most important of which are phonetic environment and position relative to the stressed syllable. Some hypotheses concerning the phonetic motivation for this kind of vowel reduction will then be discussed.