The Yarkovsky effect describes a small but significant force that affects the
orbital motion of meteoroids and asteroids smaller than $30-40$ kilometers in
diameter. It is caused by sunlight; when these bodies heat up in the Sun, they
eventually re-radiate the energy away in the thermal waveband, which in turn
creates a tiny thrust. This recoil acceleration is much weaker than solar and
planetary gravitational forces, but it can produce measurable orbital changes
over decades and substantial orbital effects over millions to billions of
years. The same physical phenomenon also creates a thermal torque that,
complemented by a torque produced by scattered sunlight, can modify the
rotation rates and obliquities of small bodies as well. This rotational variant
has been coined the Yarkovsky-O'Keefe-Radzievskii-Paddack (YORP) effect. During
the past decade or so, the Yarkovsky and YORP effects have been used to explore
and potentially resolve a number of unsolved mysteries in planetary science
dealing with small bodies. Here we review the main results to date, and preview
the goals for future work.Comment: Chapter to appear in the Space Science Series Book: Asteroids I