First-principles simulation, meaning density-functional theory calculations with plane waves and pseudopotentials, has become a prized technique in condensed-matter theory. However, it is only in the last 15 years or so that we have been able to compute the properties of condensed matter from first principles. Here we discuss the basics of the subject, give a brief review of the theory, examine the advantages and disadvatnges of its implementation, and illustrate some of the ways simulators approach problems through a small case study. We also discuss why and how modern software design methods can be used in computational materials science.