Most travellers fly into Greece. Athens is a main hub for international flights, and many European airlines fly into Thessaloniki. There are also direct flights to some of the islands, eg Mykonos, from London.
Olympic Airlines flies internally within Greece and, if you’re strapped for time, it might be worth flying straight to Mykonos or Santorini from Athens rather than endure the long ferry ride.
For many travellers, a trip to Greece is all about island-hopping. Greek Island ferry timetables can be difficult to decipher using the internet so pre-booking can be confusing. The best plan is to do some research before you leave — to ensure there are ferry connections between your destinations — but then wait until you arrive in Greece before you buy your ferry tickets. Be flexible too — ferries can be cancelled due to bad weather and can often run late.
A comprehensive bus network services mainland Greece, and each of the islands. Most towns have a central bus station.