  We have known for a long time that many of the measured white dwarf (WD)
masses in cataclysmic variables (CVs) significantly exceed the mean mass of
single WDs. This was thought to be related to observational biases, but recent
high-precision measurements of WD masses in a great number of CVs are
challenging this interpretation. We review the measured WD masses of CVs,
determine the WD-mass distribution of an extensive sample of
post-common-envelope binaries (PCEBs) that are representative for the
progenitors of the current CV population (pre-CVs) and compare both
distributions. We calculate the CV formation time of the PCEBs in our sample by
determining the post common-envelope (CE) and the main-sequence evolution of
the binary systems and define a pre-CV to be a PCEB that evolves into a
semi-detached configuration with stable mass transfer within less than the age
of the Galaxy. Possible observational biases affecting the WD-mass distribution
for the pre-CV and the CV samples are discussed. The mean WD mass among CVs is
<Mwd>=0.83\pm0.23 Msun, much larger than that found for pre-CVs,
<Mwd>=0.67\pm0.21 Msun. Selection effects cannot explain the high WD masses
observed in CVs and we here suggest two possible explanations, both of which
imply substantial revisions to the standard model of CV evolution: either most
CVs have formed above the orbital-period gap (which requires a high WD mass to
initiate stable mass transfer or a previous phase of thermal-timescale mass
transfer), or the mass of the WDs in CVs grows through accretion (which
strongly disagrees with the predictions of classical nova models). Both options
may imply that CVs contribute to the single-degenerate progenitors of Type Ia
supernovae. The number of He-core WDs found in CVs (<=10%) is roughly
consistent with the number of He-core WDs in pre-CVs (<=17+-8%), which
indicates a low value of the CE efficiency.
