In recent years, corporations have increasingly engaged in voluntary non- financial disclosure practices. This paper uses legitimacy theory to explain why firms engage in such voluntary disclosure practices, particular with respect to disclosures relating to environmental, social, and governance topics (ESG). The objective of this paper is to test and confirm the viability of legitimacy theory as a framework to describe and understand why firms voluntarily publish ESG-related information. A primary mixed method research design was utilized, comprised of qualitative research and an in-depth content analysis of existing data. The key finding was that legitimacy theory is a viable framework to describe and understand corporate social disclosure (CSD) practices, which have increased in recent years through the means of a number of legitimation strategies. This research contributes to the CSD literature by advancing legitimacy theory as a framework for examining voluntary non-financial reporting and applying this in an effort to understand the practices of a US-based law firm and textiles corporation.