The lack of labour market integration of vulnerable groups, such as refugees and other individuals with a migration background, the elderly and people with a mental or physical health impairment, receives much attention in both policy and academic circles. Academics have shown that hiring discrimination against these groups is a key factor contributing to their poor labour market integration. However, measuring discrimination is one thing, tacking it is another. To effectively combat hiring discrimination one needs to understand its driving factors. The proposed research aims to study these drivers of hiring discrimination. To this end, a worldwide unique database that combines data from 13 field experiments on hiring discrimination based on various grounds with external occupation, firm and sector data will be constructed. These data will be analysed with respect to differences in hiring discrimination by occupation, firm and sector characteristics on the one hand and with respect to the value of the main economic discrimination theories on the other hand.