Code review is a widely used quality assurance technique. From the heavy-weight and manual process that it was first introduced as code review has evolved into a more light-weight and tool-assisted process overtime to support modern software development that demands frequent delivery of source code. A large body of research on code review support exists. To help to understand the state of nature, quality, and evaluation of existing code review support, we conducted a systematic mapping study, a form of systematic literature review (SLR).