This paper explores the legal concept of the materiality of sustainability-related information under Swiss capital markets law. References to international standards, as well as EU and U.S. provisions, are also made with a view to highlighting the increasing importance of environmental, social and governance (ESG) aspects in the law and regulation of capital markets. We primarily focus on the categories of sustainability information that can be considered financially material, i.e. they have to be made available to investors so that they are able to make informed investment decisions. We assess the potential legal and regulatory consequences of the financial materiality of sustainability-related information under Swiss law. Emphasis is laid on the ad hoc disclosure regime, the prohibition of insider trading as well as the liability for disseminating false and misleading sustainability information.