Supportive care with mechanical ventilation is the cornerstone of management for acute respiratory distress syndrome (ARDS). Positive end-expiratory pressure (PEEP) is often applied in mechanically ventilated patients with ARDS to improve oxygenation; however, determining the optimal PEEP level—the pressure that maximizes clinical benefit while minimizing risks of ventilator-induced lung injury and other harms—for each patient can be challenging. Recently, transthoracic lung ultrasonography (also called lung ultrasound) has been proposed as a tool to guide PEEP determination in patients with ARDS. This paper reviews the history of use of lung ultrasound as a method to guide PEEP determination and the four published studies which compared it to other techniques of PEEP determination, such as the oxygenation and PV-curve methods.