Use a semicolon:
- To separate the independent clauses in a compound sentence not joined by a conjunction:
Only two seats were left; we needed three.
The situation is hopeful; the storm may lift soon.
- To separate two independent clauses, the second of which begins with an adverb such as however, consequently, moreover, and therefore:
We waited an hour; however, we couldn't hang around indefinitely.
- To separate elements already punctuated with commas:
Invitations were mailed to the various professors, associate professors, and assistant professors; the secretary of the department; and some of the grad students.
See also: The Semicolon: Love Child of the Comma and the Period