Genetic information is collected, sometimes stored, and given to individuals in three contexts: genetic counseling, genetic testing, and genetic screening. The purpose, assumptions, risks (including eugenic risks), and benefits of each of these undertakings will be discussed. The principle of providing genetic information will also be elaborated: 1) balancing neutrality with support, 2) confirming diagnosis, 3) deciding what, how and when to give genetic information. Finally, the clinical application of these principles to the genetic counseling for two dementias, Alzheimer's disease and Huntington's disease, will be contrasted. The issues that are salient to consultants whose family members have these two dementias are very different because of contrasting ages at onset and duration of illness and because of differences in the genes involved.