Molecular gases, that are brought to a near standstill with kinetic energies only a few 100 nK above absolute temperature zero, open up far-reaching possibilities for quantum science, ranging from quantum simulation and quantum information to quantum-controlled chemistry and fundamental physics. However, even the simplest molecules, diatomic molecules, have until recently largely eluded complete quantum control because of their multitude of quantum degrees of freedom including rotation and vibration. In her research career, Silke Ospelkaus has succeeded in demonstrating the complete quantum control of diatomic molecular systems. She has produced so-called quantum gases of ground-state polar molecules and demonstrated quantum control of chemical reactions at ultracold temperatures using the simple laws of quantum mechanics. Recently, her work has focused on quantum control of molecular collisions to enable the formation of a Bose-Einstein condensate of diatomic, polar molecules and to access the quantum world beyond diatomic molecules through controlled chemical reactions at ultracold temperatures.