  We present a sample of 30 well observed Post Common Envelope Binaries
(PCEBs). Deriving the cooling age of the white dwarfs, we show that the PCEB
population is dominated by young systems. Having calculated the orbital
evolution of the systems under the assumption of two different prescriptions
for the angular momentum loss, we find that most of the systems have not yet
completed a significant fraction of their PCEB life time. We therefore predict
the existence of a large population of old PCEBs containing cold white dwarfs
(T_eff<15000 K). Our calculations show that nearly half of the PCEBs in our
sample will evolve into a semi-detached configuration and start mass transfer
in less than the Hubble-time. These systems are thus representative for
progenitors of the current CV population. Only one of them (V471 Tau) will
evolve into a long-period (Porb>4h) CV, and a significant fraction of the
systems will start mass transfer in the period gap. Having estimated the
distances of the PCEBs in the sample, we derive a space density of ~6-30x1e-6
[pc-3], depending on the assumed angular momentum loss prescription. Taking
into account the evolutionary time scales we compute a lower limit for the CV
space density, predicted by the currently known PCEB population of ~10 x 1e-5
[pc-3]. Finally, we discuss possible observational selection effects and
conclude that the observed PCEB population is probably highly incomplete.
