This course covers the software that runs on our computers all the time and makes them usable, both by humans and by application programs. This would be reasonably simple if computers only did one thing at a time, but for the past several decades most computers have been designed to do at least a handful if not dozens or hundreds of unrelated tasks at once. The operating system is charged with giving each of these tasks the memory, CPU time, disk space, network bandwidth, etc. that it needs, without allowing it to interfere with other tasks. We'll examine many of the non-obvious issues that arise in solving these problems.