Developmental dyscalculia (DD) is a congenital deficit that affects the ability to acquire arithmetical skills. Individuals with DD have problems learning standard number facts and procedures. Estimates of the prevalence rate of DD are similar to those of developmental dyslexia. Recent reports and discussions suggest that those with DD suffer from specific deficits (e.g., subitizing, comparative judgment). Accordingly, DD has been described as a domain-specific disorder that involves particular brain areas (e.g., intra-parietal sulcus). However, we and others have found that DD is characterized by additional deficiencies and may be affected by domain-general (e.g., attention) factors. Hence "pure DD" might be rather rare and not as pure as one would think. We suggest that the heterogeneity of symptoms that commonly characterize learning disabilities needs to be taken into account in future research and treatment.