This is an additional consideration for translators, due to the uniqueness of the Greek alphabet.  We consider transliteration from a Greek source text into English under two headings:  transliteration of ancient and modern Greek words and names, and transliteration of foreign words and names.  The last might seem a superfluous category, but it is often the case that a Greek text will contain a foreign name in Greek characters, which then has to be put into the appropriate form in the English text.  This may be complicated further if the name is not English to begin with.  

For words and names transcribed from Ancient Greek, there is a well-observed set of conventions*.  For transcriptions from Modern Greek, there is less consistency. 

letter(s) transcription from Modern Greek transcription from Ancient Greek
Α a a
ΑΙ ae, e ae, e  (e regularly in US English)
ΑΥ af, av au
Β v b
Γ g, gh, y g
ΓΓ ng, g ng
ΓΚ g, ng nk
Δ d (dh) d
Ε e e
ΕΙ i ei
ΕΥ ev, ef eu
Ζ z z
Η e, i e
ΗΥ eu eu
Θ th th
Ι i i
Κ k, c c (rarely k)
Λ l l
Μ m m
ΜΠ b, mb, mp mp
Ν n n
ΝΤ d,  nd, nt nt
Ξ x x
Ο o o
ΟΙ i e, oe (e regularly in US English)
ΟΥ ou, u ou, u
Π p p
Ρ r r
word-initial Ρ r rh
ΡΡ r rrh
Σ s s
Τ t t
ΤΖ tz, dz -
Υ i, y u, y
Φ f ph
Χ h, ch, kh ch
Ψ ps ps
Ω o o
ι   υπογεγραμμένη
- generally omitted
rough breathing
- h

 * No convention of course is observed 100%.  You should always be prepared for exceptions.  

(i) names may become conventionalised outside of these equivalences, or owners of names may have their own preferred transliterations.  For example the Greek writer Καίη Τσιτσέλη uses the Latin form of her name Kay Cicellis. Manos Hatzidakis (composer) but G. Hadzidakis (linguist). 

(ii) The same Greek name or word may conventionally have two different transliterations:  Socrates (ancient Greek philosopher) but Sokratous ("Socrates" St, home of the Athens stock exchange).  Alcibiades (classical figure) but Alkiviadis (modern name).

(iii) Occasionally the English which derives from Ancient Greek is not what one might expect from this table because for instance it is a modification rather than a close transliteration.  This often happens at the ends of words:  Rhodes for Ρόδος, Aeschylus for Αισχύλος; gynaecology for γυναικολογία.   

(iv) Foreign names that were given katharevousa forms in Greek should be given their original names in Latin spelling, if the referent is the same:  Byron went into Greek as Βύρων.  As the name of the poet, it has to be rendered as Byron, but as the name of a suburb of Athens, it is Viron.  Cf also Νεύτων, Newton.