While it is customary to deal with environmental regulation and financial regulation separately as if they are not related, we depart from this tradition by demonstrating that the generic theories of regulation are applied to environmental and financial regulation. We suggest that the arguments for and against regulation are equally valid for both, and so is the principle that regulation is a response to market failure. We also argue that it is not only that environmental regulation is relevant to sustainability but that financial regulation is also relevant. We consider the issues of integrating sustainability criteria in the risk assessment and decision making of financial institutions and how financial regulation can help mobilize capital for sustainability, arguing that financial stability affects and is affected by sustainability.