  This is a brief overview of the spin physics opportunities at a high energy,
high luminosity, polarized Electron-Ion Collider (EIC). It covers measurements
of electroweak polarized structure functions, quark and gluon PDFs, TMDs, GPDs
and GTMDs. Exploiting the many possible final states allows to probe various
spin effects. Open and bound heavy quark production can be used to probe gluon
TMDs, but also color-octet NRQCD long distance matrix elements. Spin-dependent
fragmentation functions can be used too, but are also interesting in
themselves. Especially for studies of the small-$x$ and the high-$Q^2$ spin
structure the EIC will be essential.
