Membership in Sigma Xi is by nomination. 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, nearly 200 members have won the Nobel Prize and many more have earned election to the National Academies of Sciences and Engineering.

Sigma Xi has more than 10,000 members who are students. These individuals can be nominated as either associate or full members depending on their scientific research accomplishments.

Any active (dues paying) full member may nominate an individual for membership. Individuals who meet the qualifications for membership may request nomination to membership. Please see the Qualifications for Membership and the Nomination Forms for more information.