M-mode records echocardiographic data along a single scan line and displays it over time. It has two notable advantages over 2D echocardiography, which are high temporal resolution and the ability to show cardiac movement throughout the respiratory cycle. These advantages make it the best mode for certain clinical situations, including those pertaining to rapidly moving structures or those that occur over the respiratory cycle. M-mode can be used as an adjunct in the assessment of biventricular and valvular function. It can be a critical tool in the evaluation of outflow obstruction, pericardial tamponade, and IVC collapsibility/distensibility. While some echocardiographers may consider m-mode an historical entity, its advantages over 2D echo for the above-described circumstances make it a key component of the modern echocardiogram.