Mean curvature flow is the gradient flow of the volume functional of hypersurfaces. Starting from any closed hypersurfaces, the flow will develops singularities in finite time. To understand generic properties of singularities formation, Colding and Minicozzi introduced a notion of entropy of hypersurfaces, which is given by the supremum over all Gaussian integrals with varying centers and scales. In this talk, I will show that the topology and geometry of closed hypersurfaces with small entropy are simple. This is joint work with J. Bernstein.