When revising a document, I often want to highlight my changes, so my collaborators can easily see what I changed. \underline works for single words, but it puts everything in a horizontal box and long text isn't wrapped anymore. Instead, I use the package soul and \ul to underline. \usepackage{soul} Or you can highlight text using \hl. This will underline the text unless you use the color package, then it turns into typical marker-highlight. \usepackage{color}