A sentence can end with a question mark or an exclamation point, but those are special cases for special effects. Most of the sentences you write should end with periods.

Cap-and-lowercase abbreviations (such as Dr. and Jr.) also end with periods, but most all-cap abbreviations are written without them. If an abbreviation that ends with a period also ends the sentence, use only one period:

I once met Henry Louis Gates Jr.

But if such an abbreviation concludes a sentence that ends with another punctuation mark, use both the period and the other mark:

Have you ever met Henry Louis Gates Jr.?

Periods are also used in numbers with decimal points (2.0) and in URLs, or Web addresses (www.bedfordstmartins.com).