When does a patent expire?

This is not necessarily a clear-cut issue, but it is very important to understand for some types of patent searches and licensing issues, so read carefully:

1. U.S. Patents filed after June 8, 1995 expire 20 years from the date of filing.
2. U.S. Patents filed prior to June 8, 1995 expire 17 years from the date of issue, or 20 years from the first non-provisional patent application in the family - whichever is later.

Number 2 is a mouthful there. When we say "the first non-provisional patent application in the family" what we really mean (most of the time) is just "the application." But, we have to put it that way to be accurate, because Continuations-in-Part muck things up (you'll hear about them later).
Knowing how to calculate the expiration dates of patents is extremely important when conducting Infringement searches, because you only need to worry about patents that have not expired (which are referred to as being "in force").