INTRODUCTION. Forensic Neuropsychology establishes an expert valuation of the brain-injured patient (or supposed), or of the law offender people (or supposed) requiring law protection due to their illness. This discipline has a fundamental role in the study of cerebral organic syndromes, particularly in the dementias. DEVELOPMENT. The demential syndrome is common to a group of diseases as degenerative or cardiovascular disorders, brain tumors, syphilis, alcoholism or toxic factors, etc. When speaking of dementia we refer to a syndrome characterized by a progressive and global deterioration of the cognitive functions (memory, language, attention, space-temporal orientation, praxis, thinking, etc.) with preservation of the level of conscience (DSM-IV). This symptomatology affects the personality of the individual as reasonable being , as well as to his behavior and social adaptation. The decrease or loss of the intellectual and volitives abilities of the affected person of a demential syndrome, if it is permanent, also implies a change in its legal situation, since its legal capacity is altered. Therefore, it will be necessary to adopt protective measures for his person and his patrimony. Even if it is necessary, to promote a process of disability, whose sentence will be emitted by a judge, who will indicate: the degree of the mentioned disability, the trusteeship regime and who is designated as a legal tutor.