A good IAQ investigator is calm, observant, thorough, and discerning. They should be skilled in knowing what questions to ask, have knowledge of building systems, and possess a willingness to engage the public promptly and communicate results clearly to all involved parties. This guide discusses the initial investigation, a HVAC overview and inspection, considerations for special indoor environments, risk communication, and report preparation. Eleven appendices are also included, providing the reader with guidelines, checklists, and forms to assist in their investigations.