Underlining looks clumsy and reduces readability. Use bullet points to break up large bodies of text and bold or italics to emphasise text.
You can use bold to emphasise or highlight certain words. This can work well as a document or paragraph title, or in the middle of a sentence or paragraph, eg:
Bullet points can also help to make a word or sentence stand out from the rest of your text.
Avoid using too many quotation marks to emphasise words, although they can be useful to highlight a jargon term.
Avoid underlining and block capitals. These methods of emphasising text are not reader friendly, especially for those with reading disabilities. For example, leave of absence looks just as important but less threatening than LEAVE OF ABSENCE.
No matter what method you use, be consistent throughout each document when emphasising text.
See also Titles and paragraph headings and Borders and boxed text.