Plasticity is the ability of an individual organism or genotype to respond to environmental conditions, such that a given environmental state consistently induces a particular phenotype. Plastic responses are often associated with epigenetic modifications that mediate changes in gene expression and can have profound effects on fitness. While plasticity during the individual lifetime has been recognized for a long time, it is now clear that many environmental factors can induce epigenetic, physiological, or behavioral changes that have consistent effects on offspring development, and that some of these effects can persist for multiple generations. Such ‘transgenerational plasticity’ is a form of nongenetic inheritance, and studies on diverse organisms show that a broad range of environmental factors can induce transgenerational plasticity in physiological, behavioral, morphological, and life-history traits. This chapter considers the fitness effects of transgenerational plasticity and its potential to play a role in adaptive evolution. While some instances of transgenerational plasticity have clearly evolved as facultative, fitness-enhancing strategies, many examples of transgenerational plasticity are more plausibly interpreted as non-adaptive effects of stress or pathology. This chapter argues that considerable caution should be exercised in inferring adaptive function for observed transgenerational plasticity effects. Nonetheless, both adaptive and non-adaptive forms of transgenerational plasticity could contribute to adaptive evolution. Many questions and controversies remain to be resolved about the phenotypic scope, fitness consequences, mechanisms, and evolutionary causes and consequences of transgenerational plasticity.