Perineal trauma may occur during vaginal birth spontaneously or when the accoucher makes a surgical incision (episiotomy) to increase the vaginal opening. Instrumental delivery, nulliparity, birth weight, shoulder dystocia, persistent occipito-posterior position and midline episiotomy are the major risk factors. Studies with endoanal ultrasound have shown that occult obstetric anal sphincter injuries (OASIS) are common after vaginal delivery, ranging between 20 and 41 percentage. The most common techniques of primary repair following OASIS have been by end-to-end approximation with interrupted sutures and overlap repair of the external anal sphincter (EAS) with separate end-to-end repair of the internal anal sphincter (IAS). The latter showed promising results when introduced with a reduction of anal incontinence from 41 to 8% compared with matched historical controls who had an end-to-end repair. Repair of OASIS should be conducted only by a doctor who has been formally trained.