Modern Greek has only five vowels as opposed to the seven of Classical Greek.
In Modern Greek the emphasis of syllables is important to the meaning of a word.
The emphasized vowel is denoted by an accent.
 
By the transcription of Classical Greek into the Roman Alphabet there are well established rules, with only a few exceptions for example the letter "ω" Omega and "ο" Omicron.
For Modern Greek there is no one universally accepted standard for transcription into the Roman alphabet.