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Of the various muscles that make up the respiratory system, the diaphragm is the prima donna. In the past, only specialist research centers were able to estimate and challenge the effort of this muscle; this was achieved by measuring transdiaphragmatic pressure-an invasive technique involving a double-balloon probe inserted through the esophagus-or by measuring twitch pressure (ie, the pressure generated at the outside tip of the endotracheal tube). However, the prevalence of diaphragm dysfunction in critically ill patients requiring intubation can exceed 60% (at the time of hospital admission) and may rise to as high as 80% in patients requiring prolonged mechanical ventilation and experiencing difficult weaning. Although still in its infancy, modern ultrasound (US) provides a fascinating way to study the diaphragm, permitting the assessment of its excursion, thickness, and thickening. Furthermore, US enables the course of diaphragmatic function to be followed on a day-to-day basis, from intensive care admission to discharge, and it can help us understand the different causes of underlying disease: trauma, infection (eg, sepsis-induced diaphragm dysfunction), cancer, weaning problems (eg, ventilation-induced diaphragm dysfunction), etc. Today, the assessment of diaphragm dysfunction with US provides an important first step toward improving the detection of diaphragm dysfunction and as a protective and supportive strategy for its management. The purposes of this review are as follows: (1) to explore which US method is best for evaluating diaphragm function in the intensive care unit and how and when it should be used, and (2) to discuss which diseases may involve the diaphragm, and what therapies should be used.