Chemistry, the “science of matter,” is the investigation of the fabulously complex interchanges of atoms and bonds that happen constantly throughout our universe and within all living things. Computational chemistry is the computer modeling of chemistry using mathematical equations that come from physics. The field was made possible by advances in computer algorithms and computer power and continues to flourish in step with developments in those areas. Computational chemistry can be thought of as both a time-lapse video that slows down processes by a quadrillion-fold and an ultramicroscope that provides a billion-fold magnification. Computational chemists can quantitatively simulate simple chemistry, such as the chemical reactions between molecules in interstellar space. The chemistry inside a living organism is dramatically more complicated and cannot be simulated exactly, but even here computational chemistry enables understanding and leads to discovery of previously unrecognized phenomena. This essay describes how computational chemistry has evolved into a potent force for progress in chemistry in the twenty-first century.