Which iMacs can use target display mode?

Target Display Mode is supported on iMacs from 2009 to 2014 that have a Mini DisplayPort or Thunderbolt port. Specifically, the iMac models must be 27-inch (from 2009 to 2014) or 21.5-inch (from 2011 to 2014). This feature allows these iMacs to be used as external displays for other Macs. However, newer iMac models released after 2014 do not support Target Display Mode.