No, REM is not always 16px. REM is relative to the root element's font size. By default, most browsers set the root font size to 16px, but developers can change this in the CSS. For example, if you set the root element's font size to 18px, 1rem will now equal 18px instead of the default 16px. This flexibility allows you to adjust your entire design by changing just one value in your CSS.