Five to seven percent of children experience severe difficulties in learning mathematics and/or reading. Current trials that are focused on identifying biological markers suggest that these learning disabilities, known as Developmental Dyscalculia (DD) and Dyslexia (for reading), are due to underlying brain dysfunctions. One ongoing controversy concerns the extent to which arithmetic impairments are specific to DD or shared with other developmental disorders such as Dyslexia. This review explores and develops a hypothesis for cases of DD + Dyslexia. Three factors warrant consideration: (a) the behavioral factor, including definitions of the disabilities and assessment tools; (b) the cognitive factor, including whether co-occurrence of DD and other developmental disorders such as Dyslexia derive from similar or different cognitive risk factors; (c) the biological factor, including consideration of static vs. developmental neuropsychology. Better understanding of the causes of co-occurrence of DD and Dyslexia, or other developmental disorder such as Attention Deficit Hyperactivity Disorder (ADHD), can have an important influence on research that examine the two disorders, including research on therapy and etiology.