Embedded within the structure of much American environmental regulation is a distinction between the new and the existing. This distinction reflects a recurrent political challenge for environmental policymakers: whether and how to mitigate regulatory burdens when policy change upsets settled expectations and investment commitments. Environmental law often grandfathers existing products and pollution sources or provides them with other kinds of transition relief. This paper presents a survey of transition policies in environmental regulation, which is followed by a pair of short case studies drawn from the trucking and pesticide industries. These examples demonstrate that the form and extent of transition relief may be substantially influenced, first, by the cost impacts of regulatory initiatives—which are in turn shaped by the composition and competitive dynamics of the regulated industry—and, second, by path-dependent, change-resistant legal and institutional arrangements in the policy arena.