Ultralight aviation is a segment of aviation that is permitted in the United States of America by the FAA as long as certain weight, speed, and fuel capacity restrictions are observed. The governing regulation is FAR 103, and allows single seat aircraft of less than 5 gallons fuel capacity, empty weight of less than 254 pounds, to fly over unpopulated areas during daylight hours. Most ultralights can be flown safely only under ideal weather conditions.

No licence or training is required by law, but training is highly advisable. Conversely ultralights are simple to fly. A novice can be soloed with radio assistance from a qualified instructor in 2 hours for some ultralight types (powered parachutes). Other types require much more extensive instruction and practice.

There are several classes of aircraft (officially termed "vehicles" by the FAA)

• Powered parachutes - cart mounted engines with parafoil wing
• Powered hang gliders - cart mounted engines with V shaped hang glider-like wing
• Powered paragliders - backpack engines with parafoil wing
• Fixed wing ultralight - regular airplane configuration.
• Gyrocopter - rotary wing with cart mounted engine (see autogyro)

In other countries, for instance Australia, two-seat aircraft with a maximum weight of up to 544 kg are allowed in the "Ultralight" category. Training and licencing are mandatory.

Ultralights aircraft are generally called "microlight aircraft" in the UK.