Polarimetric synthetic aperture radar (POLSAR) provides additional information about the scatterers and clutter in a scene over that of single-band SAR. A fully polarimetric sensor contains four imaging channels that, when properly calibrated, can indicate the type of scatterers present. For example, it is possible to discriminate between trihedral-, dihehdral-, and dipole-like scatterers. The orientation of the scatterers can also be extracted. Based upon this additional information, hypotheses can be generated about the objects in the scene that are richer than those generated from single-band data. Combinations of transmission and reception with antennae that ideally represent orthogonal, balanced polarimetric states generate the four channels of the POLSAR system. In practice, the antenna elements are not perfect; crosstalk and imbalances exist between them, so that calibration is necessary. This paper addresses the calibration of POLSAR data, and introduces some new approaches to this problem. These include a novel gradient descent algorithm for crosstalk removal and the application of a rotating dihedral to the calibration of a sensor with receiver characteristics that are transmit-state dependent. The sensitivity of the Cloude polarimetric decomposition to varying amounts of crosstalk and imbalance in an imperfectly calibrated data set is also discussed.