Use a period to mark the end of a complete sentence (except when the sentence is a question or exclamation):
I do not like to play baseball.
From my bedroom window I see the quiet but restless night, tortured and desperate nature in pantomime.
Use a period for abbreviated names and titles
I love Mrs. Ermisch’s class.
I have an appointment with Dr. Jackson today.
When a sentence ends with an abbreviation that ends in a period, do not add an extra period:
The college is proud to announce a guest lecture by Jose Gonzalez, D.D.S.
Note: Check your style guide for determining how to handle abbreviations. Some style guides direct writers to drop the periods for common abbreviations (for example, The well known professor, Ralph Otero, PhD, will be joining our faculty).
Do not use a period for direct questions and exclamations
How much do those cupcakes cost?
Those cupcakes are expensive!
Do use a period for indirect questions
Tonya asked if it would be okay to bring her friends.
Use a period to mark a decimal point when using numerals in your sentences
The investor lost approximately 2.5 million dollars last year.
Be sure to check your style guide for direction on using periods in other particular cases, such as when using periods with parenthesis and quotations.