To monitor the mental health status of their employees, businesses rely on readymade tools that are easy to use, effective and not only assess their employees’ mental health, but also provide clues about the origins of any difficulties, thus facilitating choice of appropriate interventions. In this chapter, we describe the development and validation of an instrument for measuring mental health at work that we call Work Health Audit Instrument (WHAI) and provide an overview of its reliability and validity. We propose 18 WHAI factors that measure distinct causes for mental health difficulties at work and show that these factors relate to various relevant outcome criteria within businesses, such as work satisfaction, work engagement, irritation, and mental health. Thus, this chapter aims to give employers reasons for using this tool to measure the mental health of their employees. Additionally, we suggest that the WHAI factors can be used in further scientific inquiry, for example as a screening tool for studies on intervention effectiveness.