The aim of this paper is a historical description of the changes which affected the phonological structure of Greek over the period from ProtoIndo- European language to modern Greek_ From the purely phonological point of view, Greek can be divided as follows.
1. Proto-Greek (~1400 BC): This is the period which the changes that are unique to Greek and made it different from the other Indo-European languages took p1ace_ The changes are 1) devocalization of the aspirates, 2) loss of the labio-velars, 3) loss of /y/, 4) the change of initial /s/ to /h/.
2. Classical Greek (700BC~330BC): The changes of this period are 1) monophthongization of /ei,ou/ to /e,u./, 2) lowering of /ë, ö/ to /e, ʒ/, 3) fronting of /u, u, oi/ to /û, ü, öi/.
3. Hellenistic Koine Greek (330BC~6C AD): This period can be divided furthermore into two stages. In the first stage (4C BC~2C AD) the changes occured as follows. 1) monophthongization of / ãi, Ëi, 5i/ to / ã, e, ɔ/ respectively, 2) raising of /ε/ to / i/. And in the second stage (2C AD"-'4C AD) we see 1) monophthongization of / ai/ to /Ë/ and /öi, üi/ to /íJ./, 2) raising of /Ë/ to /l/, 3) spirantization of / ph, t h, kh/ and / b, d, g/ to / f, e, x/ and / v, ð, r / respective1y, 4) spirantization of / u/ in / au, êU/ to / f/ or /v / according to the environments, 5) loss of / h/, 6) creation of new opposition between / s/ and / z/, 7) transition of pitch-accent to stress-accent and loss of the distinction between long vowels and short vowels.
4. Modern Greek (6C AD~): In this period we have 1) merging of /ü/ with /i/, 2) change of unstressed / i/ to / y/, 3) emerging of / b, d, g/ and / ts, dz/.
Based on the above observations on the historical changes in Greek phonological structure we assume two principles which seem to govern the language change: 1) a phonological system tends to keep up the structural stability, 2) and to maintain the same number of phonemes if possible