Unicompartmental knee replacement surgery has known uneven success. It fell out of favor in the late 1970s as it was gradually replaced by total knee arthroplasty. The failures of unicompartmental replacements were due partly to the implants themselves but largely to incomplete knowledge of the principles and techniques associated with the unicompartmental knee replacement procedure. As newer implants become available, it is easy to focus on the technology at the expense of basic principles. The authors elaborate on five key principles that differentiate unicompartmental knee replacement from total knee arthroplasty.