The past decade has seen a resurgence of interest in developmental instability, reflected in fluctuating asymmetry, as a concept in evolutionary ecology. Many investigations interpret fluctuating asymmetry in populations or subsets of populations as reflecting the existence of, or at least the potential for, natural or sexual selection. However, the biological and nonbiological factors underlying the appearance of developmental instability are not well understood. For example, the ability of heterozygosity vs genomic coadaptation to have an impact on development and cause fluctuating asymmetry is still debated, though each will have important, but different, implications for the genetic structure of populations and genetic architecture of various traits. These and other issues reviewed in this chapter must be clarified in order for the concept of developmental instability to be meaningful in evolutionary and ecological studies.