i will present a recent proof of the multiplicity one conjecture in min-max theory. this conjecture was raised by marques and neves. it says that in a closed manifold of dimension between 3 and 7 with a bumpy metric, the min-max minimal hypersurfaces associated with the volume spectrum introduced by gromov, guth, marques-neves are all two-sided and have multiplicity one. as direct corollaries, it implies the generalized yau's conjecture for such manifolds with positive ricci curvature, which says that there exist infinitely many pairwise non-isometric minimal hypersurfaces, and the weighted morse index bound conjecture by marques and neves.