What are the most common mobile network operating frequencies ?

A key part of any mobile phone specification is its operating frequency bands. The supported frequency bands determine whether a certain handset is compatible with a certain network carrier.

Different countries, and different operators within countries operate on some subset of those frequency bands, hence the need for handsets to support different frequencies.

Below is a summary of the most commonly used frequencies, per world region without getting into specifics for each country:

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

For frequencies used by different US mobile carriers, see this FAQ


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


Notes:
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.