  A unique feature of non-Hermitian systems is the skin effect, which is the
extreme sensitivity to the boundary conditions. Here, we reveal that the skin
effect originates from intrinsic non-Hermitian topology. Such a topological
origin not merely explains the universal feature of the known skin effect, but
also leads to new types of the skin effects---symmetry-protected skin effects.
In particular, we discover the $\mathbb{Z}_{2}$ skin effect protected by
time-reversal symmetry. On the basis of topological classification, we also
discuss possible other skin effects in arbitrary dimensions. Our work provides
a unified understanding about the bulk-boundary correspondence and the skin
effects in non-Hermitian systems.
