Supplemental Instruction (SI) can be an efficient way of improving student success in difficult courses. Here, a study is made on SI attached to difficult first-year engineering courses. The results show that both the percentage of students passing a difficult first-year engineering course, and scores on the course exams are considerably higher for students attending SI, compared to students not attending. The study also shows that a higher percentage of female students attend SI, compared to male students. However, both genders seem to benefit to the same degree as a result of attending SI meetings. Also all students, independent of prior academic ability, benefit from attending SI. A qualitative study suggests that SI meetings provide elements important for understanding course material, which are missing from other scheduled learning opportunities in the courses.