On March 12, 1990, ICAO adopted the current airspace classification scheme

These are the ICAO definitions. Country specific adaptations (such as "two-way communications" instead of "clearance" for Class C in the US) can be discussed in the sections below, please do not place them here.

• Class A: All operations must be conducted under IFR or SVFR.All aircraft are subject to ATC clearance. All flights are separated from each other by ATC.
• Class B: Operations may be conducted under IFR, SVFR, or Visual flight rules (VFR). All aircraft are subject to ATC clearance. All flights are separated from each other by ATC.
• Class C: Operations may be conducted under IFR, SVFR, or VFR. All aircraft are subject to ATC clearance (country specific variations notwithstanding). Aircraft operating under IFR and SVFR are separated from each other and from flights operating under VFR, but VFR flights are not separated from each other. Flights operating under VFR are given traffic information in respect of other VFR flights.
• Class D: Operations may be conducted under IFR, SVFR, or VFR. All flights are subject to ATC clearance (country specific variations notwithstanding). Aircraft operating under IFR and SVFR are separated from each other, and are given traffic information in respect of VFR flights. Flights operating under VFR are given traffic information in respect of all other flights.
• Class E: Operations may be conducted under IFR, SVFR, or VFR. Aircraft operating under IFR and SVFR are separated from each other, and are subject to ATC clearance. Flights under VFR are not subject to ATC clearance. As far as is practical, traffic information is given to all flights in respect of VFR flights.
• Class F: Operations may be conducted under IFR or VFR. ATC separation will be provided, so far as practical, to aircraft operating under IFR. Traffic Information may be given as far as is practical in respect of other flights.
• Class G: Operations may be conducted under IFR or VFR. ATC separation is not provided. Traffic Information may be given as far as is practical in respect of other flights.

Classes A–E are referred to as controlled airspace. Classes F and G are uncontrolled airspace.