Dr. Fan is an observational cosmologist. The research of his group aims at answering questions such as: When did the first luminous objects appear in the Universe? How did the first generations of galaxies and quasars heat up the intergalactic gas, re-ionize the Universe and end the cosmic dark ages? He has pioneered techniques to search for the most distant quasars in the early universe, at z>6. Using these quasars, he has shown that supermassive black holes with masses up to 10 million solar masses existed within one billion years after the big bang. Meanwhile, the absorption spectra of these quasars reveal a rapid increase in cold atomic gas in the intergalactic medium, marking the end of reionziation epoch at z~6. His group is currently involved in conducting new surveys of quasars at z>7 using new near-IR large sky surveys, and studies of the co-evolution of black holes and galaxies in the early universe with facilities such as ALMA and HST. He is also involved in studying spectroscopic properties of the first generation galaxies that are responsible to reionization, using LBT, HST, and soon with JWST. In addition, he is working on a project called "MAMMOTH", a novel survey of the most massive large scale structure and protocluster environments at the peak of cosmic star formation.