Thoracic ultrasound can provide a non-invasive technique for human diaphragm functional assessment, which can be used as an alternative to traditional, more challenging, and uncomfortable methods, such as transdiaphragmatic pressure measurement, fluoroscopic sniff test, nerve conduction studies, and electromyography. The variables that can be assessed using ultrasounds are (1) the static measurement of the end-expiratory diaphragm thickness (Tdi), (2) the dynamic evaluation of the ratio of inspiratory to the expiratory diaphragm thicknesses, reported as the "thickening ratio" or "thickening fraction" (TF), and (3) the diaphragmatic excursion. The measurements of Tdi and TF are performed by placing a high-frequency linear probe at the level of the zone of apposition, while diaphragm excursion is measured using a curvilinear probe placed in the subcoastal region and recording diaphragm movements in the M-mode. Intra- and inter-observer reliabilities of the measurement of Tdi and TF are high, and ultrasound estimates of Tdi are correlated to direct anatomical measurements. Tdi can be used to monitor the evolution of diaphragm weakness. The reduced values of TF are associated with diaphragmatic paresis. Diaphragm excursion is sensitive to changes in the respiratory pattern, related to diaphragm’s volume generating capacity, and can be used to identify diaphragm weakness. In intubated patients, diaphragm excursion is related to weaning outcome.