  In the early 1980s, S. T. Yau conjectured that any compact Riemannian
three-manifold admits an infinite number of closed immersed minimal surfaces.
We use min-max theory for the area functional to prove this conjecture in the
positive Ricci curvature setting. More precisely, we show that every compact
Riemannian manifold with positive Ricci curvature and dimension at most seven
contains infinitely many smooth, closed, embedded minimal hypersurfaces.
  In the last section we mention some open problems related with the geometry
of these minimal hypersurfaces.
