Every massive galaxy harbours a supermassive black hole in its center, with masses from a few million to several billion solar masses. Their growth seems to be linked to the evolution of their host galaxies. We need to explain this connection and understand the growth history of black holes. The questions I am addressing in my research are: How are black holes growing? What is their growth history? How is their growth triggered? How is their activity connected to the evolution of their host galaxies?