United States, Canada, Mexico and most countries of South America
2G - GSM 850, GSM 1900 MHz
3G - UMTS 850, UMTS 1900, UMTS 1700, UMTS 2100
4G - LTE 700, LTE 1700, LTE 1900, LTE 2100

Europe, Asia, Australia, Middle East, Africa, and the rest of the World
2G - GSM 900, GSM 1800 MHz
3G - UMTS 900, UMTS 2100
4G - LTE 800, LTE 1800, LTE 2600

There are other less-commonly used regional frequencies in many countries, and not all 3G/4G frequencies are supported internationally.
LTE 1800 is the most common 4G international frequency.
UMTS 2100 is more common than UMTS 900 for international 3G coverage.

List of frequency bands used by major US mobile carriers:
850, 1900 MHz - 2G/3G/4G (GPRS/EDGE and UMTS/HSPA/HSPA+)
700, 1700/2100 MHz - 4G (LTE)

850, 1900 MHz - 3G (EVDO)
1700, 2100 MHz - 4G (LTE) - currently building out

800, 1900 MHz - 2G/3G/4G (PCS/EVDO/LTE)
2500 MHz - 4G (phased out WiMAX, moving to LTE)

Note: currently deploying LTE on 1900 MHz. Carrier is decommissioning its iDEN network and will be able to begin incorporating 800 MHz spectrum previously reserved for the Nextel push-to-talk service into its LTE network. Sprint plans on selling devices that also utilize Clearwire’s TD-LTE in the 2.5 GHz, adding up to tri-band LTE

700 MHz - bought in January 2014 from Verizon, to be used for 4G LTE
1900 MHz - 2G/3G (GPRS/EDGE, HSPA+ PCS)
1700/2100MHz - 3G/4G (HSPA/HSPA+/LTE)

Metro PCS (now part of T-Mobile)
1700 MHz, 1900 MHz - 3G/4G (PCS/EVDO/LTE)
Note: After the merger between T-Mobile and MetroPCS, T-Mobile is looking into shutting down MetroPCS's CDMA by 2015, freeing the AWS 1700 MHz spectrum for LTE.

US Cellular
800 MHz, 1900 MHz - 3G (PCS/EVDO)

Note that there are two common mobile network standards used in the US: GSM (Global System for Mobile) and CDMA (Code Division Multiple Acces). GSM is the more commonly supported international standard, very few countries besides the USA use CDMA based networks. While networks are constantly evolving, traditionally, AT&T and T-Mobile are GSM providers, while Sprint, Verizon, MetroPCS and US Cellular utilize CDMA.

You are making sure your phone will work in the country you are heading to. Here: http://www.gsmarena.com/network-bands.php3?sCountry=UNITED+STATES

You can also Submit Comment, post own country-based frequency band. Thank you