A unique feature of non-Hermitian systems is the skin effect, which is the extreme sensitivity to the boundary conditions. In this talk, I show that the skin effect originates from intrinsic non-Hermitian topology, by using the bulk-boundary correspondence of the equivalent Hermitian system. I also discuss a non-Hermitian counterpart of topological insulators/superconductors --- symmetry-protected skin effects. In particular, we discovered the Z2 skin effect protected by time-reversal symmetry in one and two space dimensions . We also categorized which non-Hermitian topological phases are intrinsic to non-Hermitian systems for any space dimensions and any internal symmetry [1,2].