Monaco is part of the Schengen Agreement and visitors who need a visa to enter the EU will only need one at their first entry point into the European Union states (if that is not Monaco or France). US and Canadian citizens do not need a visa to enter Monaco for stays up to 90 days. Up to date requirements can be viewed from the French Embassy in Washington DC (http://ambafrance-us.org/spip.php?rubrique=2). If you are arriving overland from France, there are no formal entry requirements.
Health and Safety
There are no vaccinations required for travel to Monaco and no specific health risks. Monaco is generally safe and the police are serious about keeping tourists and the many VIPs that visit the principality protected. The only threat is potentially petty theft. Travelers should keep their valuables in their hotel safe and generally be aware of their surroundings, especially at night. The area around the train station is seen as the highest risk for pickpockets. Never leave anything of value in your car and make sure to lock the doors when parked.