A review is given on recent topics on theories of flux pinning in superconductors. It covers the problem of estimate of elementary pinning forces of individual pinning centers interacting with flux lines and the summation problem to calculate the global pinning force density. As pointed out by Thuneberg, the elementary pinning force of pinning centers much smaller than the BCS coherence length should be reestimated by taking account of the electron-scattering effect. And calculations of the elementary pinning force of grain boundaries from the electron-scattering effect are briefly reviewed. Their dependence on the impurity parameter is opposite to the theoretical result of Thuneberg. The summation theories are reviewed and a significance of instabilities of flux lines appeared in the Labusch theory is emphasized. Then, recent two theories, the Larkin-Ovchinnikov theory and the mean field approximation theory are compared and problems contained in two theories are discussed.