Obesity is one of the serious public health problems and its prevalence has been increasing in recent years. As well as causing health problems, obesity is associated with cognitive function impairment, especially with reduced language, motor capacity skills, and attention named as executive functions. Obesity is also related to decrease in brain volume, dementia, Alzheimer's disease, sociopathic, aggressive personality traits, anxiety, and major depressive disorder. There are several mechanisms; increased vascular damage, blood pressure, C-reactive protein (CRP), cortisol, tumor necrosis factor, serum amyloid A levels explaining the relationship between obesity and cognitive functioning. Obesity generates fluctuations in levels of insulin, leptin and metabolites. These fluctuations may affect cognitive capacity by causing atrophic changes in the brain. FTO gene is also being studied to explain the relationship between obesity and cognitive function. Alzheimer's disease, Parkinson's disease, schizophrenia, and bipolar disorder are disorders commonly associated with obesity. Besides these disorders, some studies reported associations between decreases in brain volume and IQ levels and obesity. To investigate the underlying mechanisms between these two matters, more longitudinal studies are needed. In this review, we aimed to study the relationship between obesity and cognitive functioning.