High-frequency ultrasound in clinical dermatology: a review Jack Levy1 , Devon L. Barrett1 , Nile Harris1 , Jiwoong Jason Jeong2,3 , Xiaofeng Yang2,4 and Suephy C. Chen5* Abstract Background: Ultrasound was frst introduced in clinical dermatology in 1979. Since that time, ultrasound technology has continued to develop along with its popularity and utility. Main text summary: Today, high-frequency ultrasound (HFUS), or ultrasound using a frequency of at least 10 megahertz (MHz), allows for high-resolution imaging of the skin from the stratum corneum to the deep fascia. This noninvasive and easy-to-interpret tool allows physicians to assess skin fndings in real-time, enabling enhanced diagnostic, management, and surgical capabilities. In this review, we discuss how HFUS fts into the landscape of skin imaging. We provide a brief history of its introduction to dermatology, explain key principles of ultrasonography, and review its use in characterizing normal skin, common neoplasms of the skin, dermatologic diseases and cosmetic dermatology. Conclusion: As frequency advancements in ultrasonography continue, the broad applications of this imaging modality will continue to grow. HFUS is a fast, safe and readily available tool that can aid in diagnosing, monitoring and treating dermatologic conditions by providing more objective assessment measures. Keywords: High-frequency ultrasound, Dermatology, Diagnostic imaging, Ultrasonography © The Author(s) 2021. This article is licensed under a Creative Commons Attribution 4.0 International License, which permits use, sharing, adaptation, distribution and reproduction in any medium or format, as long as you give appropriate credit to the original author(s) and the source, provide a link to the Creative Commons licence, and indicate if changes were made. The images or other third party material in this article are included in the article’s Creative Commons licence, unless indicated otherwise in a credit line to the material. If material is not included in the article’s Creative Commons licence and your intended use is not permitted by statutory regulation or exceeds the permitted use, you will need to obtain permission directly from the copyright holder. To view a copy of this licence, visit http://creativeco mmons.org/licenses/by/4.0/. Introduction Dermatology is among the rare specialties of medicine in which the organ of interest can be readily examined and biopsied. As a consequence, histology has become the gold standard for many dermatologic diagnoses. It has also allowed for clinicopathologic correlation to guide therapeutics and advance prognostics [1]. In the last few decades, a number of imaging modalities have been introduced to augment clinical exam or obviate the need for histology [2]. Some examples include total body digital photography (TBDP), dermoscopy, refectance confocal microscopy (RCM), optical coherence tomography (OCT), and ultrasound. In this review, we discuss how ultrasound fts into the landscape of skin imaging. We provide a brief history of its introduction to dermatology, explain key principles of ultrasonography, and review its use in characterizing normal skin, common neoplasms of the skin, dermatologic diseases and cosmetic dermatology. History of ultrasound in medicine Sonography dates back to the late 1700s when two Italian zoologists discovered the acoustic orientation sense of bats [3]. Nearly a century later, the Curie brothers presented the piezoelectric efect of sound waves on natural quartz [4]. It was their observation that a force applied to the crystal produced an electric discharge. Conversely, an alternating electric current applied to a crystal produces vibrations that generate high-frequency sound waves called ultrasound [4, 5]. Over a half-century later, the technique was adapted by both the American military as