  Studies on hadron interactions from lattice QCD are reviewed. The $S$-wave
$\pi\pi$ scattering lengths of the I=0 and I=2 channels are extracted from
various lattice determinations of low energy constants in $N_f=2$ chiral
perturbation theory. The results agree with each other and agree also with
other non-lattice estimates. Recently the $P$-wave $\pi\pi$ scattering phase
shift for the I=1 channel has been calculated. A preliminary estimate of the
$\rho$ meson decay width from the phase shift is consistent with the
experimental value. Two approaches to potentials between hadrons are discussed.
One is a method using static quarks to define the distance between two hadrons.
The other is a method to define a potential from a wave function of two
hadrons. An application of the latter to the nucleon-nucleon ($NN$) potential
turns out to reproduce qualitative features of the phenomenological $NN$
potential such as attraction at long distance and repulsion at short distance.
Theoretical issues of this approach are also discussed.
