They call it Judgement Day, and on Saturday more than 60,000 fans will watch Wales' four rugby regions in action at the Principality Stadium in Cardiff.
The showpiece of the domestic game in Wales - in its fourth season - will see Cardiff Blues tackle Ospreys followed by Newport Gwent Dragons against Scarlets.
Get in the mood with some classic Welsh derby moments.
Here, referee Nigel Owens and TMO Derek Bevan get in a pickle as Blues play Ospreys in 2014.
Owens is in charge when the two sides meet again on Saturday - could lightning strike twice?