1- To separate the elements in a series (three or more things)."He hit the ball, dropped the bat, and ran to first base." 2- Use a comma + a little conjunction (and, but, for, nor, yet, or, so) to connect two independent clauses, as in "He hit the ball well, but he ran toward third base." 3- to set off introductory elements, as in "Running toward third base, he suddenly realized how stupid he looked." You will find more here : http://grammar.ccc.commnet.edu/grammar/commas.htm http://www.grammarbook.com/punctuation/commas.asp