The FAA requires ADS-B Out capability in the continental United States, in the ADS-B rule airspace designated by FAR 91.225:
There is a graphical depiction and additional information on the FAA website.
At this time, only the United States is allowing the 978UAT datalink for ADS-B Out. If you plan to fly in ADS-B airspace outside of the United States, a 1090ES datalink—using a Mode S Extended Squitter transponder—will be required. Because the list of countries with ADS-B Out requirements and proposals is growing, we strongly recommend equipping with 1090ES if you plan to fly internationally. Remember that for countries without a mandate, there is no ADS-B Out requirement.
Australia. 1090ES required for all IFR operations. Foreign-registered aircraft equipped with transponders are exempted below FL290 until June 6, 2020.
Canada. Nav Canada has stated its intent to require 1090ES ADS-B Out equipage with antenna diversity, but rulemaking has not been completed. Nav Canada will use the Aireon space-based ADS-B system, of which it is an owner.
Europe. 1090ES required for IFR aircraft with a MTOW exceeding 12,566 pounds or maximum cruise airspeed faster than 250 KTAS. Mandatory for new-production aircraft, and must be retrofitted into all aircraft by June 7, 2020. (Dates have slipped from original timetable.)
Hong Kong. 1090ES required in all airspace, FL290 and above.
Indonesia. 1090ES required, FL290 and above.
Mexico. 1090ES required in Class A, B, and C airspace; Class E above 10,000 feet msl; and other specified airspace, including Class E airspace over the Gulf of Mexico, at and above 3,000 feet msl within 12 nm of the Mexican coast.
Singapore. 1090ES required on specified airways.
South Africa. 1090ES required April 1, 2020, for all IFR operations and for any aircraft first registered or replacing its transponder and flown in Class A, B, D, or E airspace.
Sri Lanka. 1090ES required within the Colombo Terminal Control Area (TMA), FL290 and above.
Taiwan. 1090ES required in all airspace, FL290 and above.
Vietnam. 1090ES required on specified airways.