Maryland has the same entry requirements as the rest of the US outlined by the State Department at http://travel.state.gov/visa/temp/types/types_1262.html. Most European nations, Japan and Singapore are part of the Visa Waiver Program. However, all foreign travelers will need to register with the Electronic Systems for Travel Authorization (ESTA) prior to entering the country (https://esta.cbp.dhs.gov/)
Health and Safety
Traveling to Maryland is generally safe. The state is serious about tourism and the major attractions are heavily patrolled by police. As always, common sense will help avoid bad situations. Stay away from dark, quiet areas off the beaten path alone, especially at night. Vehicles should always be locked and valuables not left in plain sight.
Most accidents occur outdoors, especially in water. Visitors should take care when enjoying water sports and hire appropriate guides or join tours if they are inexperienced. Enjoying the mountains safely requires preparation. Always know where the closest ranger station is or where help can be found. Always take extra water and provisions and check weather forecasts.