All travelers to Mongolia require a visa, except a national of the USA who can stay as a tourist without a visa for up to 3 months. Passports must be valid for at least 6 months from date of arrival.
All foreign nationals wishing to stay longer than 30 days are required to register with the local police within 10 days of arrival.
Visas are generally valid for 30 days from date of entry (and 3 months from date of issue); and can be extended in Mongolia by a maximum of 30 days.
Types/Costs are:
Single-entry: US$65
Double-entry: US$105
Multiple-entry: US$135
Single-transit: US$65
Double-transit: US$75
Multiple-transit: US$85
US citizens that need to renew, add pages, change name, or just get a new US passport, can Apply for a New US Passport here.