Techniques involved in measurements of solar magnetic field are based on the observation and interpretation of the polarization signal in suitable Fraunhofer lines. This paper discusses some facts about polarized radiation, the physical concepts underlying such measurements, and the techniques involved. Particular attention is given to the operating principles of polarimeters used in magnetic field measurements; the physical components of polarimeters; and the polarization phenomena, such as absorption, emission, and scattering, in a spectral line. Consideration is also given to the transfer of polarized radiation in a magnetized medium, line formation in a magnetic field, transfer equations for the Stokes parameters in a magnetized atmosphere; and solutions of the transfer equations and magnetic field measurements. Finally, methods for magnetic field measurements in unresolved structures and in prominences are described.