Visas are issued on arrival at Prince Said Ibrahim Airport for 30-50 euros, payable in euros or US dollars. Your passport must be valid for at least six months. There is no need to arrange anything in advance for most nationalities.
Money -- the local currency is the Comorian Franc (KMF), which is pegged to the euro at a fixed rate, making exchange rates predictable. ATMs exist in Moroni and the larger towns but are not always reliable, and card acceptance is very limited. Bring euros in cash -- they are widely accepted, with change given in KMF. Banks in Moroni can exchange money but expect slow service. Budget travelers should note that the Comoros is not particularly cheap for what you get, partly because so much has to be imported.
Language -- Comorian (Shikomori) is the everyday language, with four variants spoken across the islands. Arabic and French are both official languages. French is widely understood and is the language of education and government. English will get you almost nowhere outside of the very occasional hotel that caters to international visitors. A few phrases in French are essential.
Safety -- the Comoros is generally safe for travelers in terms of crime, but political instability is a reality. Protests can flare up in Moroni, sometimes with little warning, and have occasionally turned violent. Avoid demonstrations and stay aware of the political situation before and during your trip. Petty theft occurs, particularly in Moroni -- do not flash expensive items. The US State Department has at times issued elevated travel advisories for the Comoros.
Health -- malaria is present on all islands, so take prophylaxis and use repellent. Medical facilities are extremely basic. There is a hospital in Moroni but it lacks equipment and supplies that travelers from developed countries would consider standard. For anything serious, evacuation to Reunion, Nairobi, or South Africa would be necessary. Bring a well-stocked first aid kit and any prescription medications you need.
Electricity is 220V with European-style two-pin plugs. Power cuts are frequent on all islands.
Internet and phone -- mobile coverage exists in main towns, and SIM cards are cheap and easy to buy. Data speeds are slow. Wi-Fi in guesthouses is unreliable at best.