Penalising a process is to modify its distribution with a limiting
procedure, thus defining a new process whose properties differ somewhat
from those of the original one. We are presenting a number of examples
of such penalisations in the Brownian and Bessel processes framework.
The Martingale theory plays a crucial role. A general principle for
penalisation emerges from these examples. In particular, it is shown in
the Brownian framework that a positive sigma-finite measure takes a
large class of penalisations into account.