  We present a model-independent analysis of $K^+\to\pi^+\ell^+\ell^-$ and
$K_S\to\pi^0\ell^+\ell^-$ decays, including $K\to 3 \pi$ unitarity corrections
and a general decomposition of the dispersive amplitude. From the existing data
on $K^+\to\pi^+ e^+ e^-$ we predict the ratio $R=
B(K^+\to\pi^+\mu^+\mu^-)/B(K^+\to\pi^+ e^+ e^-)$ to be larger than 0.23, in
slight disagreement with the recent measurement $R = 0.167 \pm 0.036$.
Consequences for the $K^\pm \to\pi^\pm e^+ e^-$ charge asymmetries and for the
$K_L\to\pi^0 e^+ e^-$ mode are also discussed.
