Proton is a composite particle, so its spin (= 1/2) is expected to be equal to the sum of spins of the constituents of the partons (quarks and gluons) it consists of. However, European Muon Collaboration stated in 1987 that there was a big discrepancy between the proton spin and the sum of spins of the partons in Deep-Inelastic Scattering (DIS). This result was so unexpected that it was named Proton Spin Puzzle aka Proton Spin Crisis. Later, experiments at RHIC confirmed the existence of the proton spin problem in DIS. I discuss the current state of that problem and overview various theoretical ways to solve it.