ERS | monograph Introduction Christian B. Laursen1,2,3, Najib M. Rahman4,5,6 and Giovanni Volpicelli7 The sinking of RMS Titanic in 1912 led to the invention of a range of devices as a means of improving the detection of icebergs. During World War I, the technique was further developed into an active sound device using quartz for the detection of submarines. In the following decades and during World War II, the technique was further developed and named sonar (SOund Navigation And Ranging) [1]. The principles and technologies that lead to the development of sonar were also noticed amongst physicians. In 1940, Gohr and Wedekind suggested the use of reflected sound for diagnosing tumours, effusions and abscesses [2]. Dussik was the first to report the clinical use of reflected US as a medical diagnostic tool in his exploration of whether visualisation of intracranial structures and ventricular measurements was possible with US waves [3]. Despite Gohr and Wedekind’s initial suggestions and studies published by other authors, the use of TUS as a clinical tool was for many years considered to be limited to the assessment of pleural effusion [2, 4–6]. A description of the use of TUS as a tool for the assessment of horses with respiratory diseases challenged this dogma. Rantanen reported the use of TUS for the assessment and diagnosis of such conditions as lung consolidation, atelectasis, abscesses, pleural effusion, empyema and pneumothorax in horses [7]. Furthermore, Rantanen’s paper contains descriptions of vertical and horizontal reverberation artefacts as well as the concept of movement of the “pleural blades” during respiration in normal lungs, and its absence if pneumothorax is present [7]. These signs and artefacts were later to be considered key concepts in TUS [8]. The subsequent studies during the 1990s by researchers such as Targhetta and Lichtenstein lead to the birth of TUS as an essential diagnostic modality in the assessment of patients with known or suspected disease in the chest [9–14]. A milestone was reached when the first international consensus conference producing evidence-based recommendations for point-of-care LUS was published in 2012 [8]. This document achieved the great aknowledgement of becoming the most cited article of the top 50 published in Intensive Care Medicine, the journal of the European Society of Intensive Care Medicine it was even more highly cited than other very Copyright ©ERS 2018. Print ISBN: 978-1-84984-093-4. Online ISBN: 978-1-84984-094-1. Print ISSN: 2312-508X. Online ISSN: 2312-5098. Correspondence: Christian B. Laursen, Dept of Respiratory Medicine, Odense University Hospital, Sdr. Boulevard 29, 5000 Odense C, Denmark. E-mail: Christian.b.laursen@rsyd.dk 1 Dept of Respiratory Medicine, Odense University Hospital, Odense, Denmark. 2 Centre for Thoracic Oncology, Odense University Hospital, Odense, Denmark. 3 Institute for Clinical Research, SDU, Odense, Denmark. 4 Oxford Centre for Respiratory Medicine, Oxford University Hospitals NHS Foundation Trust, Oxford, UK. 5 Oxford Respiratory Trials Unit, Nuffield Dept of Medicine, University of Oxford, Oxford, UK. 6 Oxford NIHR Biomedical Research Centre, Oxford, UK. 7 Dept of Emergency Medicine, San Luigi Gonzaga University Hospital, Torino, Italy. https://doi.org/10.1183/2312508X.10000518 xiii