This chapter describes obstetric anal sphincter injuries. This topic will cover basics of all degrees of fresh perineal tears (Obstetric anal sphincter injuries or OASIS) with particular emphasis on 3rd and 4th degree perineal tears. Out of total, 85% of women who have a vaginal birth have some form of perineal trauma. The morbidity associated with perineal injury and repair is a major health problem. Trauma can be deliberate to widen the passage of outer birth canal or spontaneous while conducting a difficult vaginal delivery or an operative vaginal delivery. Anatomy of perineum, mechanism of anal sphincter injuries, classification of perineal tears, classification of 3rd and 4th degree perineal injury, prediction and prevention, identification of obstetric anal sphincter injuries, repair of perineal trauma, repair of oasis, recommended material and technique for 1st and 2nd degree tears or episiotomy, suturing of 3rd and 4th degree perineal tears, choice of suture materials, and postoperative care are also discussed in this chapter.