A neurobiological brain disorder (NBD), also referred to as a neurobiological disorder (NBD), is a term used to refer to a host of disease or other biological processes affecting the brain. Any condition that affects the functioning of the brain and that may be caused by genetic, metabolic, endocrinological, auto-immune, infectious agents, toxic agents, extreme environmental factors triggering biological changes, or other biological factors can be described as an NBD. They do not include psychological or emotional problems people have such as situational depression and low self-esteem. However, emotional problems can develop from the severity of the symptoms experienced by individuals with any major disease process, including NBDs. This does not negate the possibility that in some cases, extreme psychological stress can be a trigger resulting in a biological disease process. However, the resulting biological problems in an NBD, are not themselves, of a psychological nature.

Many of the symptoms resulting from the various biological disorders are classified in the International Statistical Classification of Diseases and Related Health Problems (ICD-10) under "Mental and behavioural disorders", which include underlying etiologies from organic (problems resulting directly from damage or disease affecting the structure of the brain) as well as those resulting from more subtle or unknown etiologies afftecting the brain itself.

The term NBD may be deprecated by some groups advocating that all problems of the brain are controlled by the mind (and thus within the power of the mind to heal the brain --see dianetics), and by individuals focusing on mental problems of a psychological or emotional nature.

Many disorders affecting the brain have a neurobiological basis. The resulting symptoms are sometimes referred to as a neuropsychiatric disorder.

Many different biological processes are implicated in the development of these disorders. Some of these processes, illnesses and symptoms considered to be neurobiological brain disorders are: Traumatic brain injury, Hypothalamic dysfunction, brain symptoms from diseases of the endocrine system (pituitary, thyroid, adrenals), Thyroxine(T4) transport (see Transthyretin, some Mitochondrial diseases, autism spectrum disorders, learning disabilities, Myelinization problems, psychiatric symptoms stemming fromMultiple sclerosis, Epilepsy, Migraines and other neurological and autoimmune etiologies, schizophrenia, Bipolar disorder, Pervasive developmental disorder (PDD), Premenstrual dysphoric disorder, Tourette syndrome, PANDAS, viral infections, parasitic infections, autoimmune diseases, nutritional deficiencies, Parkinson's disease with dementia, obsessive-compulsive disorder (OCD), Alzheimer's disease, as well as some forms of major depressive disorder (MDD), anxiety, Panic Disorder and attention deficit hyperactivity disorder (ADHD).