Risk assessment is an essential component of genetic counseling and testing, and Bayesian analysis plays a central role in genetic risk assessment. Bayesian analysis allows calculation of the probability of a particular hypothesis, either disease or carrier status, based on family information and/or genetic test results. Genetic risk should be assessed as accurately as possible for family decision making. Additional information, from the pedigree and/or from genetic testing, can often dramatically improve the accuracy of genetic risk assessment. We illustrate herein the application of Bayes' theorem and describe important basic principles in genetic risk assessment.