Skarpi, it depends on whether the wearer is a "traditionalist" or a "modernist" , usually "traditionalists" only wear underpants when taking part in Highland sports or in public dancing displays, this is because during these activities the sporran ( the flat poch worn hanging in front of the kilt)is usually pivoted onto the hip ( so it doesnt swing out and beat your partner black and blue on the turns) does not keep the front of the kilt in place as it usually does and preserve ( somewhat) the modesty of the wearer, but otherwise they probably go "Commando"(that is wear nothing at all underneath) Micca . Kilt wearer( see Getaway pics)