As the industry shifts focus from indoor air quality (IAQ) to indoor environmental quality (IEQ), the need arises for its field consultants and inspectors to meet the demands of assessment, evaluation and control, particularly in established buildings with a longer history of operation. In the past, IAQ assessments have often remained isolated to specific complaint areas, focused on conditions of concern occurring in the immediate work area at the time of the assessment, and quite commonly provided insufficient comprehensive analysis of the cause and origin of conditions that impact the indoor environment. Given the vast array of factors that contribute to healthy and productive IEQ, a proper assessment truly lies in the intersection of IAQ, building and material sciences, and mechanical system design and operation. These assessment projects best serve building owners and managers by being both expansive horizontally, taking a building as a whole from its exterior skin down to its inner core of structural and mechanical systems, and also vertically by providing ongoing consultation and project management that guides the client from an experience of problematic conditions and loss of environmental control, through repairs, renovations, remediation and verification, and back to the point of proper control and healthy IEQ. A very effective approach is a hybrid assessment that combines industrial hygiene and indoor air quality expertise, beginning inspection at the point of occupant concerns or building material damage, with mechanical system and ventilation expertise that begins the evaluation with an audit of the design, operation and condition of the equipment and automation programs, all supplemented by long-term environmental monitoring and trending. Typically, the findings of these two fields come together in the middle to complement each other and provide a robust understanding of the mechanisms and processes that cause conditions of concern and impact IEQ.