The news that Pope John Paul evidently engaged in self-mortification — whipping himself as a form of penance — has sparked some curiosity about this ancient practice.

The BBC has some background:

Flagellation is the beating or whipping of the skin, most often on the back, and often drawing blood, as a bodily penance to show remorse for sin.

It was a widespread practice in some parts of the Catholic ministry up to the 1960s but is uncommon today, says Professor Michael Walsh, a Catholic historian.

Flagellation is acted out for symbolic purposes during penitential processions during Lent’s Holy Week in Mediterranean countries, he says, as a reminder that Jesus Christ was whipped before the Crucifixion.

But in some countries like the Philippines, this re-enactment of the suffering of Jesus Christ – called the Passion play – can take a more extreme form and can draw blood.

For others self-flagellation is a more private expression of faith.

It is thought to have come to prominence in Western Europe in medieval times around 600 to 800 AD as an extreme version of bodily penance, says Professor Lewis Ayres, a Catholic theologian at Durham University.

Early Christians believed that the notion of bodily penance allowed control of the body and emotions in order to focus more fully on worshipping God.

The practice continued in what Mr Ayres calls “the more conservative Catholic orders” well into the 20th Century and is still probably practised by a “tiny minority” today.

There’s more at the link. You can also find a more detailed analysis at the Catholic Encyclopedia (courtesy New Advent).