For the past years, my colleagues and I have developed a one-semester course that teaches scientists and engineers the "common core" of modern software development. Our experience shows that an investment of 150 hours-25 of lectures and the rest of practical work-can improve productivity by roughly 20 percent. That's one day a week, one less semester in a master's degree, or one less year for a typical PhD. The course is called software carpentry, rather than software engineering, to emphasize the fact that it focuses on small-scale and immediately practical issues. All of the material is freely available under an open-source license at www.swc.scipy.org and can be used both for self-study and in the classroom. This article describes what the course contains, and why.