Ultrasound of the diaphragm in critically ill patients has become a diagnostic technique of emerging interest among clinicians and scientists. The advantages include that it is widely available, non-invasive and examination can be performed after relatively short training and at low costs. It is used to estimate muscle mass by measurement of muscle thickness and diagnose weakness by the assessment of diaphragm movement during unassisted breathing. Thickening of the muscle during inspiration has been used to quantify force generation. The enthusiasm that surrounds this topic is shared by many clinicians and we agree that ultrasound is a valuable tool to screen for diaphragm dysfunction in intensive care unit (ICU) patients. However, in our opinion much more studies are required to validate ultrasound as a tool to quantify breathing effort. More sophisticated ultrasound techniques, such as speckle tracking imaging are promising techniques to evaluate respiratory muscle function in patients, including the critically ill.