Over the last two decades, angle-resolved photoemission spectroscopy has emerged as one of the most important experimental tools used in the study of complex materials, with the high-temperature superconductors as one of the most important examples. The improved resolution and carefully matched experiments have turned this technique into a sophisticated many-body spectroscopy. The information from this technique has been crucial in our understanding of high-temperature superconductors. In this paper we briefly introduce the progress in this field.