In the American Psychological Association citation format, used primarily by those working in the social sciences, involves brief in-text citations combined with full reference lists, but the use of underlining is never mandatory. It can be used optionally, however, for web links, as well as in situations where you would normally use italics.
It is acceptable to underline the web links or URLs to online sources from which you acquired an article or journal. Otherwise, the entry is the same as one using italics. An example of this method: Hanna, G., Patterson, M., Rollins, J. and Sherman, A. (2011). The arts and human development: Framing a national research agenda for the arts, lifelong learning and individual well-being (underlined). National Endowment for the Arts Office of Research and Analysis. Retrieved 17 June 2013 from http://www.nea.gov/pub/TheArtsAndHumanDev.pdf
Note that there is no period following directly afterward.
Reference List Book or Journal Titles
You can also underline the titles of books or journal instead of italicizing them. You must underline the entire title and ensure you are consistently using the same format throughout your writing. When underlining the title of a journal or periodical, you must continue underlining through the volume number, but not the issue number nor the page numbers. An example: Tressel, G.W. (1994). Thirty years of "improvement" in precollege math and science education (underlined). Journal of Science Education and Technology (underlined) 3 (2), 77-88.
Scientific, Technical or New Terminology
Underlining scientific or highly technical terms you wish to emphasize in your writing is also allowed in the APA citation format. This can include the names of species, such as homo sapiens, scientific processes or novel terms, such as intercultural co-opetition, you have created to explain some innovative portion of your research. These terms should be underlined in the first usage in your writing, while subsequent uses should appear without an underline.
Although underlining and italicizing are considered interchangeable for certain portions of APA citations, the fifth edition of the APA style manual established a strong preference toward italics over underlines. While these instances of underlining are nevertheless still considered acceptable, you must be aware that underlining should never be used with in-text citations, headings throughout your writing, such as chapter or section titles nor for emphasis of a particular point. For example, the corresponding in-text citation for the references listed in the previous sections should appear as (Hanna et al., 2011) and (Tressel, 1994).