This chapter discusses properties of the probes for RF magnetic field measurements and in particular, the factors limiting the measurement accuracy. Our considerations are limited to a probe consisting of a circular loop antenna loaded with a detector of a shaped frequency response. However, the majority of results are fully applicable for Hall-cell probes, magneto-optic probes, those with a magneto-diode, and for other designs, especially when considering averaging of the measured field upon the surface of the measuring antenna (probe). Many of the considerations are similar in character to those presented in Chapter 4, and some are concerned with the magnetic field measurement specificity.