The talk will be a general report on the eta invariant which was introduced by Atiyah, Patodi and Singer in their work on spectral asymmetry. The eta invariant arises as boundary correction term in the Atiyah-Patodi-Singer index formula for manifolds with boundary and is defined in terms of the eigenvalues of the tangential part of the differential operator near the boundary. Since the invention of the eta invariant, many interesting relations with other fields of mathematics have been discovered. As examples, we mention special values of L-series, the holonomy of determinant line bundles, Witten's 3-manifold invariants and topological QFT. The talk will give an overview over recent developments connected with the eta invariant.