This simple question causes horrible confusion, and the many different answers you get from different people don't help. The United States' illogical and complicated rules can be hard to follow and keep straight.

Fortunately, what the U.S. rules lack in logic they have in consistency. Commas and periods always belong inside the quotes.

Wrong: Many say, "The amount of carbon dioxide in the atmosphere is increasing".
Right: Many say, "The amount of carbon dioxide in the atmosphere is increasing."

Colons and semicolons go outside quotation marks.

Wrong: "An apple a day keeps the doctor away;" but what if you happen to like doctors?
Right: "An apple a day keeps the doctor away"; but what if you happen to like doctors?

All other punctuation—dashes, exclamation points, parentheses, and question marks—go inside the quotation marks only if they are part of the original quote. Otherwise, they belong outside the marks, because they are therefore part of the entire sentence.

Wrong: "Be careful"! parents often warn their children.
Right: "Be careful!" parents often warn their children.

Wrong: Did John say, "The boss will be back next week?"
Right: Did John say, "The boss will be back next week?"

The "British rule" (from the Oxford Guide to Style) is a bit more logical. Punctuation marks go inside the quotes if part of the original quote, and outside otherwise. The Chicago Manual of Style declares this option acceptable when it's needed for accuracy.

Occasionally acceptable: Many say, "The amount of carbon dioxide in the atmosphere is increasing".
(This would be saying that there is no period after increasing in the source quoted. If the original source did have a period at that point, this example would be wrong.)

Each set of rules applies to both single quotation marks (') and double quotation marks ("). Whichever set of rules you use for something, make sure to be consistent. Otherwise, the resulting hodgepodge of punctuation will look accidental.

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