Fluorescent penetrant inspection (FPI) is the most widely used global inspection method, playing a particularly important role in aviation. Given the contribution that reliable implementation of the FPI process can make to flight safety, the US Federal Aviation Administration (FAA) has funded a programme to assess the FPI parameters and their role in effective detection of typical flaws. Iowa State University’s Center for NDE (CNDE) has led a team that includes many industry partners from the aviation industry during this six-year programme. The industry partners have provided guidance and prioritisation input, and in many cases access to internal data, samples or use of facilities to support the programme. The focus of the programme has been on quantitative assessment of FPI performance using a combination of data gathering methods, which has included indication luminance as measured using a photometer, UV-A indication appearance using a fluorescence stereomicroscope, and in some cases the more traditional probability of detection (POD) study. Numerous studies have been completed, although results of developer studies will be the focus of this paper. Choice of indication development parameters starts with the selection of the developer form. All four forms have been evaluated during this programme: dry powder, water-soluble, water-suspendible, and nonaqueous wet developers. The results of the developer studies are presented below along with key ‘lessons learned’.