Should I put one space or two after a period?

Ok, this answer is easy:

Unless you are using an old fashioned typewriter, you should only use one space after a period.

Look at any professional publication you respect. Look at every book you have. Look in every magazine. You will find only one space after the periods.

The reason why (older) people were taught to put two spaces after a period is because back before computers, people wrote things on typewriters. Typewriters used something called monospace fonts, that is, fonts in which every letter was the same width. In order to make a pleasing space after a period, two spaces were needed when using a typewriter.

However, once the personal computer came into existence, every person had the power of a professionally typeset (i.e., non-monospace font) at their fingertips. Two spaces after a period were no longer required.

If your coworkers use two spaces after a period, just ask them to stop. If you want to fix their document, you can just do a "find and replace" for ".  " (that is, a period with two spaces after it) and replace with ". " (a period with one space after it).

The one instance where I would make an exception is in any technical sort of writing where part of the requirements you are given are specifically to add two spaces after a period. I would argue that anyone with such a rule is stuck in the past, but rather than put your career/script/legal brief etc. in jeopardy, just roll with it. (Side note: I ran into this situation once. I just wrote the document correctly (with one space after the period), then did a find and replace when I was finished to fix the formatting. I also gently informed the person who required this archaic practice about modern typesetting, and I never had to do this again.)

Before this turns into a major debate like my post about not putting the MBA designation on your business card or in your email signature, just remember that you don't have to take my word for it.

Listen to the Chicago Manual of Style, which is of the view  that there is "no reason for two spaces after a period in published work."

Listen to the MLA Handbook, which only uses a single space after a period in all of its examples.

Listen to Typography For Lawyers (a personal favorite), which says to "put exactly one space after any punctuation." (I particularly like how in this link they show you the difference between monospaced and non-monospaced fonts).

Listen to the always cool Slate Magazine, which punches you right in the face with the following statement: "typing two spaces after a period is totally, completely, utterly, and inarguably wrong."

Finally, listen to The Economist, which has a macro that automatically strips a second space from after a period if one should find its way onto the magazine's online content management system.

The Economist does leave the door open a crack for some debate, taking the view that spaces after a period are a style choice rather than a hard and fast rule. I think they were a bit too kind in that approach. It is also a "style choice" not to wear bellbottoms or a stovepipe hat, which, if you use two spaces after a period, you might as well do because you are truly living in the past.

Feel free to comment if you disagree- the internet is a beautiful thing!