A period and one space. In my defense, I have to say that I chose Courier font for my dissertation. Old habits die hard.
When I pointed out that they were doing it wrong—that, in fact, the correct way to end a sentence is with a period followed by a single, proud, beautiful space—the table balked. “Who says two spaces is wrong?” they wanted to know.
Typographers, that’s who. The people who study and design the typewritten word decided long ago that we should use one space, not two, between sentences.