An increasing number of hereditary neurodegenerative diseases, including autosomal-dominant Alzheimer disease (AD), familial autosomal-dominant frontotemporal dementia (FTD), and heritable Lewy body disease (LBD) have been defined at the molecular level in recent years, making it possible to determine the genotype before the onset of symptoms. The identification of deterministic genes for these common adult-onset genetic diseases is moving the field of genetic counseling toward a new and challenging direction. With the identification of genes associated with AD and FTD, there is considerable interest in the clinical application of genetic information in genetic counseling and testing. Progress in the genetics of dementing disorders and the availability of clinical tests for practicing physicians therefore increases the need for a better understanding of the multifaceted issues associated with genetic testing. The aims of this systematic review are: (1) to underline the need to consider a genetic etiology of AD, FTD, and LBD; (2) to provide clinicians with information necessary to effectively translate genetic diagnosis into clinical practice; and (3) to highlight gaps and uncertainties in the field which will need to be addressed by future research.