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A protracted or arrested active phase of labor represents a difficult dilemma of clinical obstetrics, as the situation has to be handled rapidly. Both unsuccessful operative vaginal delivery or a late Cesarean delivery could lead to serious injuries both to the mother and the fetus. In such clinical circumstances, the need for a “transparent” pelvic canal and objective and precise imaging measurements for the evaluation of labor outcome, instead blind and subjective clinical assessment, is nowhere more pertinent. The use of intrapartum ultrasound is not meant to change the classic algorithm of a prolonged or arrested labor monitoring, but to provide objective and reliable evaluations of the traditional crucial features. In this chapter we discuss the role of ultrasound in optimizing the outcome of prolonged labor.The findings regarding a prolonged first stage of labor in nulliparous demonstrated a better reliability of ultrasound evaluations and a good correlation between sonographic measurements and labor duration and outcome. A head to perineum distance less than 40 mm, an angle of progression of more than 110–120°, a progression distance more than 35 mm and an angle of direction of more than 105° represent good predictors for vaginal delivery and should encourage labor continuation.In the prolonged second stage of labor, head to perineum distance and angle of progression are significantly associated with the median duration and success operative vaginal delivery. Lack of head descent during pushing (angle of progression increase < 15°, head to perineum distance increase < 2mm or lack of the fetal head descent with respect to the infrapubic line) were associated with longer duration of operative vaginal delivery and vacuum extraction failure.Generally, the studies on prolonged labor were designed to evaluate the value of a certain parameter. We believe that it would be beneficial to investigate the value of multivariate analyses that include several parameters following the model of other feto-maternal successful predictions, as the combined test. Perhaps more accurate predictions could be obtained with the inclusion of other useful ultrasound determinations, such as asynclitism, pubic angle evaluation or head deflection estimation. As a matter of fact, when a clinician estimates the prognosis of an obstructed labor, takes into account all these features of the labor mechanism, and the role of ultrasound evaluation is to objectively elucidate by all means the delivery settings.