Do drivers have to participate in practice sessions?

In order to participate in qualifyingQualifying (Quali) is a one-hour session used to determine pole position and the order the cars will line up behind... There's more... and start the race, a driverOK, so we all know what the driver is, right. I've always liked the description that F1 drivers are like... There's more... must participate in at least one free practiceFree Practice consists of three sessions (FP1, FP2 & FP3) held prior to qualifying. It is referred to as 'free... There's more... session. 

However, it is unusual to see a driver sit out a practice session as there are only three one-hour sessions for them and the team to familiarise themselves with the track and dial the car in.