Twice-exceptional students are students with both a high potential for mathematical abilities and educational special needs. They are particularly at risk for neither their potential nor their disabilities being recognized. Students who work below their potential are called underachievers. This chapter discusses whether the special learning conditions of twice-exceptional students need a differentiated approach than what is usually applied for underachievement. Furthermore, by means of examples of affected students, the implications for learning processes are illustrated. The focus is put on mathematical giftedness occurring together with learning disabilities (LD), attention deficit disorders (ADD), attention deficit disorders with hyperactivity (ADHD), and autism spectrum disorders (ASD).