In this talk, I will introduce the techniques of radio polarimetry and Faraday rotation for the study of cosmic magnetic fields. Faraday rotation is a birefringent effect caused by magnetised plasma along the line of sight, which we measure using the frequency-dependent rotation of the plane of linearly polarized light from radio galaxies (i.e. synchrotron emission). Radio galaxies can be observed throughout the majority of the history of the Universe and are thus excellent beacons for measuring the properties of the cosmic web and their evolution with cosmic time. In particular, I will highlight recent results from the Low Frequency Array (LOFAR) radio telescope. LOFAR is the world’s premier low-frequency radio telescope, providing exceptional RM precision, in addition to unrivalled angular resolution, sensitivity and image fidelity, which facilitates the reliable identification of the host galaxy through comparison with optical and infrared data (from which one can then determine the redshift). Our recent work shows how these capabilities are allowing us to transform our understanding of cosmic magnetic fields and are providing a new way to study the properties of filaments and voids of the cosmic web in general.