If you've done everything you can to find all of the necessary information, and some of it is just missing, you can use these abbreviations. 

Be careful - make sure the information isn't there before you use these placeholders.  For example, if you are looking at a webpage, and some key information, like the organization that published it, is missing, check the main page on the site before you give up.  Make sure that you have correctly identified the source and the source type.

 

Citation Style

Missing information

Abbreviation or rule

MLA

Author is missing

Start your reference with the title.

The source has no page numbers/ is not paginated.

No pag.

The source has no publication date.

n.d.

The source has no publisher.

n.p.

APA

Author is missing.

Substitute title for author.

The source has no date.

n.d.

The source has no title.

Describe the source in square brackets. [ ]