Impairments in patients with dementia and other disorders affecting cognition may have a negative impact on their capacity to provide consent to treatment or to participation in research. A growing literature confirms that even patients with mild cognitive impairment may experience decrements in decisional abilities, findings that are more pronounced still in the early stages of dementia. However, most patients with mild dementia probably remain competent to provide a valid consent to treatment or research, and even some patients with moderate dementia may retain capacity in particular circumstances. Clinical evaluation of decisional competence has been augmented by structured approaches, including reliable instruments that may be used in the clinical setting. To avoid needlessly depriving patients of their right to make health care decisions, evaluations should be designed to maximize patient performance. However, when substitute consent is necessary, state laws generally provide a range of options, including advance directives and familial consent.