Darwin was one of the first biologists interested in the developmental failure of perfect symmetry, what is now known as fluctuating asymmetry (FA). FA is the random left–right size differences in bilateral traits designed by selection to be perfectly symmetrical. Thus at the population level genuine FA (as opposed to directional or antisymmetry, neither of which are the focus of this chapter) should be normally distributed with an asymmetry mean of zero. Arguably, L. Van Valen helped to facilitate the modern scientific study of fluctuating symmetry, despite a long history of its use. FA is an indicator of underlying developmental instability which displays the inability of an organism to reach an adaptive end-point when experiencing adverse environments. FA is often used as an indicator that structural features are developing under environmental and genetic stress; essentially FA may be an important example of a disruption in homeostasis.