InExplanatory item response models|(  the explanatory item response modeling (EIRM) approach (De Boeck and Wilson, 2004); (De Boeck and Wilson, 2016) item responses are seen as dependent variables to be explained by independent variables, which can be subject-specific but also item-specific variables. The focus of the modeling is on the impact of the explanatory variables and person parameters are considered as accounting for the heterogeneity of responses. The approach allows to investigate which variables determine the responses and is related to differential item functioning considered in Chap.  10 .