The semimajor axes of asteroids up to about 20 kilometers in diameter drift as a result of the Yarkovsky effect, a subtle nongravitational mechanism related to radiation pressure recoil on spinning objects that orbit the sun. Over the collisional lifetimes of these objects (typically, 10 to 1000 million years), orbital semimajor axes can be moved by a few hundredths of an astronomical unit for bodies between 1 and 10 kilometers in mean radius. This has implications for the delivery of multikilometer near-Earth asteroids, because the Yarkovsky drift drives many small main-belt asteroids into the resonances that transport them to the Mars-crossing state and eventually to near-Earth space. Recent work has shown that, without such a drift, the Mars-crossing population would be depleted over about 100 million years, a time scale much smaller than the age of the solar system. Moreover, the Yarkovsky semimajor axis mobility may spread in an observable way the tight semimajor axis clustering of small asteroids produced as a consequence of disruptive collisions.