Membership in Sigma Xi is by invitation. Those who have shown potential as researchers are invited to join as associate members. Full membership is conferred upon those who have demonstrated noteworthy achievements in research. Each year the Society initiates more than 5,000 new members. Over the course of the Society's distinguished history, more than 170 members have won the Nobel Prize and many more have earned election to the National Academies of Sciences and Engineering.
Member: Any individual who has shown noteworthy achievement as an original investigator in a field of pure or applied science is eligible for election or promotion to full membership in the Society.
Associate Member: Any individual who has, through initial research achievement in a field of pure or applied science, shown aptitude for research which is expected in due course to lead to the fulfillment of the requirements for full membership, is eligible for election to associate membership.
Noteworthy Achievement: Noteworthy achievement in research specified for election or promotion to full membership ... must be evidenced by publication, patents, written reports or a thesis or dissertation, which must be available to the Committee on Admissions if requested. If the work is "classified," it shall be assessed for the Committee by a "cleared" Member. Membership in the Society is neither linked to the possession of any degree nor contingent upon belonging to some other organization.
Research Aptitude: Research aptitude specified for election to associate membership ... must be evidenced by independent investigation ordinarily resulting in a written report, which must be available to the Committee on Admissions if requested. If the work is "classified," it shall be assessed for the Committee by a "cleared" Member. Associate membership is offered to encourage young investigators with promise to continue careers in research. In making the nomination, both the nominator and the seconder attest to the nominee's potential for future promotion to Member.