The Greek alphabet remains to this day almost unchanged.
Modern Greek is written with the same alphabet as Classical Greek of antiquity.
Only a few diacritic signs have changed or disappeared from Modern Greek.
 
Here is a brief overview of the Greek Alphabet.

* (at the end of a word)

As mentioned the Greek alphabet has hardly changed.
As a result Modern and Classical Greek are sometimes identically written, but are strongly differentiated by pronunciation.
This is because changes in the pronunciation of Greek from Antiquity to today have not been reflected in the written language.