Postdoctoral scholars are a critical labor source within academia, and a postdoc appointment has become a prerequisite to most academic science careers. A small but growing body of literature examines postdocs' career experiences and outcomes. The present research adds to this literature by seeking to understand how STEM postdocs exhibit science identity, a concept that may shape their persistence in academic science careers. Findings highlight how postdocs perform science and seek legitimacy as scientists and suggest the need for postdocs' supervisors and institutions to develop more realistic expectations for the postdoc role and stronger support for the postdocs they employ.