In this thesis new methodologies are developed for the detection and systematic study of radio sources in the early universe. This allows us to gain a deeper understanding of the formation and evolution of galaxies, the activity of supermassive black holes, and the final phase transition of our universe: the epoch of reionization. Using the Low Frequency Array (LOFAR) telescope, this thesis systematically investigates the low radio frequency properties of quasars, the brightest non-variable objects in our cosmos, in the first billion years after the Big Bang. Through the discovery of new radio quasars in the early universe and subsequent studies, this thesis shows the diversity within the quasar population and highlights the importance of multi-wavelength observations for our comprehension of the formation and evolution of active supermassive black holes and their impact on the surrounding environment.Large scale structure and cosmolog
