Eligibility guidelines for membership to the Sigma Xi organization are described in the Sigma Xi Constitution as quoted below:

Section 3. Requirements for Election or Promotion to Membership.

Member (Full 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 by a chapter or the Committee on Qualifications and Membership.

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 by a chapter or the Committee on Qualifications and Membership.

Definition of Terms (from the Sigma Xi Bylaws):
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 Qualifications and Membership 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 Qualifications and Membership 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, the nominator attests to the nominee's potential for future promotion to full member.

Non-discrimination Policy:
Neither Sigma Xi, nor any chapter of Sigma Xi, shall discriminate against candidates for election to membership on the basis of race, religion, sex, age, national origin, or disability.