From predicting the properties of nanotechnological devices to the structural stability of small proteins and dynamics of water. Atomistic computer simulations of matter based on solving quantum mechanical equations is an interdisciplinary area that touches physics, chemistry, and a part of biology. By calculating the electronic structure of an arrangement of atoms, and at the same time predicting the forces acting on the individual nuclei (which are themselves quantum particles), it is possible to calculate a range of properties of known and unknown materials and molecules in a computer. I will illustrate some of the successes of these theories, from predicting the properties of nanotechnological devices to the structural stability of small proteins and dynamics of water.