D evelopmental stability is the ability o f organisms to buffer against the random varia=tion that arises spontaneously as a consequence o f stochastic variation in the cellular processes that are involved in the development o f morphological structures. Its con= verse, developmental instability, is the imprecision that leads to morphological variability even when genetic and environmental conditions are kept constant, and can be conveniently mea= sured as the random left-right differences o f bilaterally symmetric organisms. This chapter demonstrates that the genetic control o f developmental stability is intimately connected with nonadditive genetic variation o f the morphological traits o f interest. Dominance and epistasis have also been shown by empirical studies to play an important role in the genetic architecture o f developmental stability. A brief review o f some mechanisms that generate stochastic varia= tion in gene expression suggests that nonadditive genetic variation is also an important factor for the origin o f developmental noise.