Although a substantial amount of research has been conducted on differential item functioning in testing, studies have focused on detecting differential item functioning rather than on explaining how or why it may occur. Some recent work has explored sources of differential functioning using explanatory and multilevel item response models. This study uses hierarchical generalized linear modeling to examine differential performance due to gender and opportunity to learn, two variables that have been examined in the literature primarily in isolation, or in terms of mean performance as opposed to item performance. The relationships between item difficulty, gender, and opportunity to learn are explored using data for three countries from an international survey of preservice mathematics teachers.