  Physical reason behind the mass-shift of vector mesons ($\rho$, $\omega$,
$\phi$) in nuclear matter is discussed in the Walecka model and in QCD sum
rules. Using analytic formulas for the mass-shift valid at low densities, it is
shown that the energy dependent part of the sels-energy (wave function
renomalization) in medium is a main source for the negative mass-shift, while
the energy independent part (the plasma frequency) has relatively a small
effect. Future experiments in GSI ($\pi^- + A \to X + \omega$ ) and KEK-PS ($p
+ A \to X + \phi$ ) to detect the twin peak structure of vector mesons in
$e^+e^-$ spectrum are reviewed. Also, it is proposed a new possible experiment
using a $S (strangeness) = 1$ vector meson $K^{*}(892)$ with its radiative
decay $K^{*+}(892) \to K^+ + \gamma$.
