  We use QCD sum rules to calculate the hadronic matrix elements governing the
rare decays $B \to K \ell^+ \ell^-$ and $B \to K^* \ell^+ \ell^-$ induced by
the flavour changing neutral current $b \to s$ transition. We also study
relations among semileptonic and rare $B \to K^{(*)}$ decay form factors. The
analysis of the invariant mass distribution of the lepton pair in $B \to
K^{(*)} \ell^+ \ell^-$ and of the angular asymmetry in $B \to K^* \ell^+
\ell^-$ provides us with interesting tests of the Standard Model and its
extensions.
