Use a period to end all sentences except direct questions or genuine exclamations. Also use periods in abbreviations according to convention.

To end sentences

Most sentences should end with a period. A sentence that reports a question instead of asking it directly (an indirect question) should end with a period, not a question mark.

If a sentence is not a genuine exclamation, it should end with a period, not an exclamation point. (See also 38c.)

In abbreviations

A period is conventionally used in abbreviations of titles and Latin words or phrases, including the time designations for morning and afternoon.

note: If a sentence ends with a period marking an abbreviation, do not add a second period.

Do not use a period with US Postal Service abbreviations for states: MD, TX, CA.

Current usage is to omit the period in abbreviations of organization names, academic degrees, and designations for eras.