Utilization of high-frequency ultrasonography, also referred to as ultrasound biomicroscopy or ultrasound backscatter microscopy, to visualize microscopic scale structures began in the 1930s with the advent of acoustic microscopes and laser scanning microscopes. instruments were designed to offer “acoustic” contrast in excised, thin tissue specimens, without the necessity for time-consuming cross-sectioning and staining procedures necessary in traditional histopathological examination. While these microscopes were not widely accepted as alternatives to optical microscopy, the use of high fre­quency, pulsed transducers for in vivo longitudinal, high resolution visualization of tissue began to emerge in the 1980s (Chapters 4 and 6). view than optical systems, these high frequency probes were initially utilized in clinical applications for ophthal­mology and dermatology. Using a 100 MHz probe, Pavlin et al,6 were able to acquire images of Schlemm’s canal, cornea, iris, ciliary muscles, and retina in intact eye cross­sections at axial depths approaching 4 mm and with a lateral resolution of 20 µm. Similarly, 20–30 MHz probes have been utilized to accurately assess extent and thick­ness of skin melanomas and basal cell carcinomas in large patient cohorts.