Shimming denotes the technical procedure to improve the homogeneity of the magnetic field in the MRI system. This presentation will give an overview about why the magnetic field is inhomogeneous at all, and what the consequence is. Passive and superconducting shims as technical means to improve the shim in every MRI system are described. Practical information on when shimming is needed, and what the operator can do to optimize the shim of their MRI system will be given as well. Moreover, latest developments like high-order shim systems, shim arrays and dynamic shimming will be presented.