The two most common propulsion systems for air-to-surface missiles are rocket motors, usually with shorter range, and slower, longer-range jet engines.
Some Soviet-designed air-to-surface missiles are powered by ramjets, giving them both long range and high speed.
A major advantage of air-to-surface missiles for ground attack by aircraft is the standoff distance they provide: missiles can be launched from a distance without coming within range of the target's air defences.
Most air-to-surface missiles are fire-and-forget from a standoff distance, allowing the attacker to withdraw without approaching further after launch.
There have been examples of air-launched ballistic missiles (Air Launched ICBM, GAM-87 Skybolt), but they are rare.