Introduction: The diaphragm plays a significant role in the ICU setting, as it forms a crucial part in sustaining spontaneous breathing and ability to wean from mechanical ventilation. A lot of research has been conducted to find new approaches and parameters for its assessment. Out of these, ultrasonography has become a popular option due to its many advantages. In this review, the aim was to provide an extensive summary on our current knowledge on ultrasonography of the diaphragm. Methods of application, feasibility, limitations and future perspectives will be discussed. Methods: This is a narrative review. A thorough search in PubMed was conducted to find all relevant articles. Filters were applied to sort out studies that were conducted in animals or patients younger than 18 years. Results: Overall, ultrasonography presents a promising imaging technique to assess diaphragm function. Diaphragm motion can be used to detect diaphragm dysfunction, paralysis and patient ventilator asynchrony, while thickness and the thickening fraction can be used to predict weaning outcomes and monitor diaphragm strength. Limitations are potential inaccuracy inherent to the ultrasound machine and variability between observers, assisted modes of ventilation (motion cannot be interpreted) and controlled modes of ventilation (ultrasound not applicable). Discussion: While a range of studies are available that demonstrate ultrasonography as a viable tool to assess the diaphragm, only a few have been conducted in sufficiently large and representative patient groups. Further research is needed for validation of those ultrasound derived data and conclusions. Until then, it is key to consider the limitations of ultrasonography carefully. Nevertheless, ultrasonography does provide valuable information and should be used to monitor the diaphragm. © 2018, Netherlands Society of Intensive Care. All rights reserved.