Poor memory and difficulty with attention and concentration are among the most common symptoms of patients with functional somatic syndromes. This chapter discusses the neuropsychological research to determine the objective severity of these deficits and the relationship with psychiatric symptoms. Poor memory and difficulty with attention and concentration are among the most common symptoms of patients with functional somatic syndromes. The influence of depression on the cognitive performance of patients with chronic fatigue syndrome was first assessed in a controlled study performed by researchers from the State University of New York at Stony Brook and the Albert Einstein College of Medicine, New York. Somatic symptoms were recorded with a somatization subscale of a known symptom checklist. Neuropsychological testing of motor and mental speed relied on the completion of reaction time tasks. An emerging common denominator for difficulties with memory, attention, and concentration appears to be somatic burden of illness, which in turn correlates with depression and anxiety.