In every election cycle a certain number of representatives retire or run for other offices leaving behind open seats for their parties to defend. Here are the number of retirements in recent years:
| Year | Democrats | Republicans |
| 2018 | 18 | 34 |
| 2016 | 8 | 20 |
| 2014 | 16 | 25 |
| 2012 | 23 | 20 |
| 2010 | 17 | 20 |
| 2008 | 6 | 27 |
| 2006 | 9 | 18 |
This cycle, 0 Democrats, 0 Republicans and 1 Libertarian are retiring at the end of their current term.
Here are the announced retirements, sorted on PVI (Cook Partisan Voting Index).
Any seat between R+5 and D+5 is a swing district that has to be fought over, even more so
if occupied by the "wrong" party.