Ultrasound-guided peripheral nerve block has been gaining popularity in the community of anesthesiologists. Despite its well-known clinical benefits, inconsistent success in block has been one of the major limita- tions of conventional peripheral nerve block. Recent development in ultrasound technology enables us to see nerves, needles and surrounding structures in real-time. We expect that ultrasound imaging may overcome the shortcoming of conventional technique. In this article, we introduce some evidences that support the clinical benefits of ultrasound-guided peripheral nerve block, including performance time, quality, onset and duration of sensory blockade, cost analysis and patient satisfaction. However, ultrasound-guidance may be even more dangerous without appropriate imaging of target nerve and surrounding structures, accurate localization of needle tip and assessment of local anesthetic spread around the target nerve. These problems will be solved by future guidelines, teaching program and further advancement in ultrasound technology.