It's better to cite than not to cite.
Let's say it's the theory of relativity. You would list it as Einstein's theory of relativity (it might need to be capitalized; please check), but if you are relying on an interpretive work by someone else (say, Stephen Hawking), or quoting another work, then cite the secondary work as well.
E. g. Einstein's theory of relativity as interpreted by Stephen Hawking in Relativity: It's Not Just Your Cousins
, page 67, June 2017 etc etc (all of the stuff you do for AP
or MLA style