Most of the physical phenomena around us are of many-body in nature, such as phenomena arising from atoms, molecules and condensed matter systems. The wave function of such systems is a complicated object containing a large amount of information, which usually makes exact or analytical solutions impractical or even impossible. The many-body methods which have been used successfully worldwide can be grouped into two categories: the methods of quantum chemistry and the methods of density functional theory. These are very different methods both at fundamental as well as application daises. In this seminar, I will provide a broad picture of these two very different many-body methods with the help of cutting edge research problems. In the first half of my talk, I will discuss the important physics that one can probe from a single atom (or ion) by applying a suitable many-body tool. In the second half, however, I will consider a more complicated system -- perovskite ferroelectrics -- as an example.