Summary form only given. Optically sensed probes designed to accurately measure electromagnetic fields are described. The probes use passive electrooptic modulators of Pockels cell or integrated optics designs to transfer the electromagnetic signal to an optical carrier propagating in a fibre-optic link. This approach eliminates the need for active components or power sources in the probe head. It minimizes the distortion of the field being measured and is immune to EMI (electromagnetic interference) pickup on the lines leading to the signal processing electronics. Several probe systems that have been fabricated at the National Institute of Standards and Technology over the last few years are described. They include an isotropic probe consisting of three orthogonal dipoles, a probe for EMP measurements up to 200 kV/m, a probe using an integrated optics modulator for frequencies to above 3 GHz, and a probe to simultaneously measure both the electric and magnetic fields and their relative phases. >