  Lattice calculations of the proton spin components is reviewed. The lattice
results of the quark spin from the axial-vector current matrix element at $\sim
0.3- 0.4$ is smaller than those from the constituent quark models. This is
largely due to the fact that the vacuum polarization contribution from the
disconnected insertion is negative. Its connection with the anomalous Ward
identity is clarified and verified numerically. This resolves the contentious
issue in the `proton spin crisis'. The glue spin and angular momentum are found
to be large and there is notable contribution from the quark orbital angular
momentum. Renormalization, mixing and normalization of the quark and glue
angular momenta are discussed. With sufficient precision, they can be compared
with more precise experimental measurements when the electron-ion collider
facility is available.
