Tardis:Period

Periods are naturally used at the end of sentences to bring closure. However, because of varied use across the English speaking world, the use of periods with abbreviations must be arbitrarily decided her on our wiki.

In keeping with our general preference for British English usage, the following is "correct" here:
 * As mentioned at T:HONOUR, no period shall be used if the first and last letters of a personal title are used to form the abbreviation of that title. A period shall be used if the usual abbreviation does not include these letters.  Hence, it's Mr Smith but Rev. Arnold Golightly.
 * without a period: Mr, Mrs, Ms, St
 * with a period: Rev., Sen., Rep., Prof.
 * In the use of acronyms, no period shall be used, owing to the modern British standard which has essentially caved into American usage. Yes, UNIT was once U.N.I.T., but nowadays it's not.  Hence NASA, CIA, FBI, etc.

You should put no more than two spaces after a period, as is explained elsewhere.

Periods and quotation marks
One of the more problematic cases with periods is how they're used with respect to quotation marks. What matters here is whether you're quoting a whole sentence or just a phrase.

When quoting a full sentence, the ending punctuation of the quotation overrides the need for the overall sentence to have a period.

If quoting a fragment that ends a sentence, the period always goes outside of the final quotation mark.

Periods precede decimals
Separate integers from fractional values with periods, not commas. Prices, for instance, should be written as $100.07, not $100,07.