Traditional teaching depicted the cervix as competent or incompetent; however, current evidence suggests that cervical "competence" is rather one anatomic component of a more complex spontaneous preterm birth syndrome that also involves the uterus and the chorioamnion. Although some women whose poor obstetric history suggests a dominant cervical factor actually have physical examination evidence of poor cervical integrity, the vast majority of women who are diagnosed clinically with cervical insufficiency have normal cervical anatomy, both between pregnancies and in early gestation. If and when the mechanical integrity of the cervix is compromised, other pathways to prematurity may be stimulated, appearing clinically as the preterm birth syndrome. Thus, the term "cervical insufficiency" may be evolving into a convenient label to describe a more complex, but poorly understood process of pathological premature cervical ripening. This chapter presents evidence-based guidelines for the selection of patients who would reasonably benefit from cerclage.