  I review various phenomena associated with mass-accreting white dwarfs (WDs)
in the view of supersoft X-ray sources. When the mass-accretion rate is low
(\dot M_{acc} < a few \times 10^{-7} M_\sun yr^{-1}), hydrogen nuclear burning
is unstable and nova outbursts occur. A nova is a transient supersoft X-ray
source (SSS) in its later phase which timescale depends strongly on the WD
mass. The X-ray turn on/off time is a good indicator of the WD mass. At an
intermediate mass-accretion rate an accreting WD becomes a persistent SSS with
steady hydrogen burning. For a higher mass-accretion rate, the WD undergoes
"accretion wind evolution" in which the WD accretes matter from the equatorial
plane and loses mass by optically thick winds from the other directions. Two
SSS, namely RX J 0513-69 and V Sge, are corresponding objects to this accretion
wind evolution. We can specify mass increasing WDs from light-curve analysis
based on the optically thick wind theory using multiwavelength observational
data including optical, IR, and supersoft X-rays.
  Mass estimates of individual objects give important information for the
binary evolution scenario of type Ia supernovae.
