Based on the development of a more refined conception of legitimacy than has been used in prior audit/assurance and sustainability accounting research, this paper analyses how the legitimation processes adopted by sustainability assurance practitioners in a large professional services firm have co-evolved with and impacted upon their attempts to develop this form of assurance practice - particularly the construction of assurance statements. The analysis reveals a complex and interdependent interplay between attempts at securing pragmatic, moral and cognitive legitimacy with three key constituencies - clients who commission the sustainability assurance services; (socially constructed) non-client users of the assurance statements; and the firm's internal Risk Department that approves the wording of assurance statements. Securing these types of legitimacy is shown to require the adoption and alignment of varying legitimation strategies according to the constituency practitioners seek to influence. Developing pragmatic legitimacy with clients depends on establishing moral legitimacy with non-client users of assurance statements while securing moral legitimacy with non-client users is contingent on acquiring pragmatic legitimacy with the firm's internal Risk Department. The practitioners' legitimation strategies are underpinned by a commitment to opening up dialogue within the assurance process which is evident in their engagement with potential users of assurance and their efforts to expand assurance statement content and encourage user influence over what is assured. This provides a counterpoint to Power's (1994, 1999) concerns about the tendency for new assurance forms to restrict debate and dialogue and reveals a rare empirical domain where Power's (2003b) call for more customised and informative narratives in assurance reporting is being heeded.