The characteristics of photonic probes that give them the potential to measure pulsed electromagnetic fields more accurately than has been possible in the past are discussed. An overview of system design is presented with particular emphasis on the transfer functions of appropriate antennas and electrooptic modulators. Noise sources which limit the sensitivity to low signals are discussed and used to determine the noise equivalent field (NEF). The characteristics of some probes developed by the US National Institute of Standards and Technology (NIST) are presented. The NEFs of these probes are about 0.1 V-m/sup -1/-Hz/sup -1/2/ and are about 10/sup 3/ times the theoretical limits. For a 1-Hz bandwidth their dynamic ranges are on the order of 70 dB. Optical guided wave technology holds considerable promise for fabricating modulators with a wide variety of operating parameters that can improve probe performance and extend their applications to a wider region of the electromagnetic spectrum. >