Experience is a feature added for kerbals as of version 0.90.0 'Beta'.  Kerbal pilots, engineers and scientists gain experience for completing a variety of achievements. Experience grants additional abilities, making crew members grow more useful over time.
Orbiting, planting flags, and escaping from the gravity of planets and moons grants various amounts of experience depending on the planet. Each Kerbal can only gain the experience once per planet, so planting multiple flags or repeatedly orbiting a planet do not add provide any further experience for the same Kerbal. Additionally, experience from a given planet or moon is only granted for the most valuable achievement, so orbiting the mun (3 XP) and planting a flag on it (5 XP) will only grant 5 XP, not 8. Note that when planting flags, each Kerbal in the mission must plant a separate flag to gain the experience for doing so. If only one Kerbal in the mission plants a flag, only that Kerbal will gain experience for having done so.
|Planet/Moon||Fly By (XP)||Orbit (XP)||Flight (XP)||Land (XP)||Plant Flag (XP)||XP Multiplier|
As Kerbals gain experience, they gain levels in their specialization. Kerbals start at level 0, and can reach level 5.
Benefits of experience
As Kerbals level up, they gain new abilities. There are usually new abilities at every level, but some of the levels do not have new abilities, and none of the specializations gain new abilities when going from level 4 to level 5.
At level 0, pilots can stabilize a ship and allow SAS. At level 1, they can aim a ship prograde or retrograde. At level 2, they can orient a ship to the normal, antinormal, radial in, and radial out vectors. At level 3, they can orient a ship towards a target, away from a target, or towards a maneuver. They gain no further abilities with additional levels.
Level 0 engineers have no special abilities. At level 1, they can repair parachutes. At level 2, they can repair lander legs. At level 3, they can repair wheels. They gain no further abilities with additional levels.
Scientists improve the amount of Science gained from returned science parts and vessels. At level 0, there is no bonus. The bonus is 5% at level 1, 10% at level 2, 15% at level 3, 20% at level 4, and remains 20% at level 5.