OSTEP ("oh step"), or the "the comet book," represents the culmination of years of teaching intro to operating systems to both undergraduates and graduates at the University of Wisconsin-Madison Computer Sciences department for nearly 20 years. The book is organized around three concepts fundamental to OS construction: virtualization (of CPU and memory), concurrency (locks and condition variables), and persistence (disks, RAIDS, and file systems). The material, if combined with serious project work and homeworks, will lead students to a deeper understanding and appreciation of modern OSes. The authors, Remzi and Andrea Arpaci-Dusseau, are both professors of Computer Sciences at the University of Wisconsin-Madison. They have been doing research in computer systems for over 20 years, working together since their first graduate operating systems class at U.C. Berkeley in 1993. Since that time, they have published over 100 papers on the performance and reliability of many aspects of modern computer systems, with a special focus on file and storage systems. Their work has been recognized with numerous best-paper awards, and some of their innovations can be found in the Linux and BSD operating systems today.