External ultrasound has achieved an important niche in the spectrum of diagnostic imaging modalities. Its real-time capability, ease of use, and relative low cost have brought it to prominence as an important diagnostic tool. Medical ultrasound imaging, driven by advances in technology and by clinical needs, continues to improve its diagnostic capabilities. Key technologies for ultrasound development are new transducers, advances in signal processing algorithms, and increased computer power. Although external ultrasound image quality continues to steadily improve, certain clinical limitations such as organ access and tissue attenuation have spurred the development of more invasive scanning techniques. Endorectal, endovaginal, and transesophageal probes provide better access to--and provide superior images for--the prostate, uterus and heart. Intraluminal ultrasound is an emerging field of imaging, employing miniature, high-frequency probes which can be inserted into arteries to monitor interventional procedures. To put these developments into perspective, this manuscript reviews the capabilities and limitations of existing ultrasound technology and discusses the impetus for future developments.