This document provides guidance on conducting complaint investigations and managing a complaint system. It discusses the key principles of an effective complaint management system, including visibility, accessibility, and continual improvement. It also outlines the steps to take in an investigation, such as preparing a complaint management plan, gathering evidence through interviews and documents, consulting parties to find a resolution, and ensuring decisions are upheld. Finally, it provides tips for interviewing parties as part of an investigation, emphasizing the need for impartiality, preparation, and managing expectations.