The Comoros has one international airport: Prince Said Ibrahim International Airport (HAH), located about five kilometers north of Moroni on Ngazidja. It is a small airport with limited facilities.
From East Africa, Ethiopian Airlines flies from Addis Ababa and Kenya Airways flies from Nairobi. These are the most frequent connections and the easiest way in from most of the world -- connect through Addis Ababa or Nairobi from wherever you are starting. Ethiopian Airlines has the most departures.
From the Indian Ocean region, Air Austral connects Moroni with Reunion, and there are flights to and from Mayotte (Dzaoudzi), which is actually the most frequent route in and out of Moroni. If you are coming from Madagascar, there are connections through Mayotte or Nairobi.
From Europe, there are no direct flights. The most practical routing is through Addis Ababa or Nairobi, or through Reunion and Mayotte if you are coming from Paris (Air Austral flies Paris-Reunion regularly).
Visas are available on arrival at the airport and cost 30-50 euros, payable in euros or US dollars. You need a passport valid for at least six months. The process is slow but straightforward.
There are no passenger ferries connecting the Comoros to mainland Africa or Madagascar on any regular schedule. Occasional cargo boats make the crossing, but this is not a practical option for most travelers.