Ultrasound has developed over the past 50 years into a major diagnostic imaging modality, complementing CT, MRI and nuclear imaging. Major applications of ultrasound today include cardiovascular, abdominal organs, muskloskeletal, small parts, and OB/Gyn. Increased clinical usage of ultrasound has been driven by technological advances that exploit the following advantages compared to other modalities: real-time (especially important for heart and blood flow), safe due to non-ionizing radiation, portable, and low cost. Basic ultrasound modes include B-mode that images the acoustic reflectivity of tissue structures and Doppler that measures blood velocity. Recent advances include harmonic imaging that improves image quality by exploiting the nonlinear behavior of high-amplitude ultrasound propagation in tissue or micro-bubble contrast agents, and code technology that circumvents traditional resolution / penetration tradeoffs. Future directions for ultrasound are at the intersection of clinical needs (image quality, new applications, and increased productivity) and major technological trends (miniaturization, SW), which include miniaturized systems and probe components, improved image quality, new imaging parameters, and 4D imaging.