The morbidity risk assesses the risk of manifesting illnesses having a variable age at onset. We reviewed the conceptual derivation and critical assumptions of various methods for estimating morbidity risk by classifying them into two approaches. One approach uses an age at onset distribution as a weighting system. A second approach uses methods from survival analysis. Because survival methods estimate the morbidity risk and age at onset distribution simultaneously, they are preferable to weighting methods. Among weighting methods, Strömgren's estimator or Risch's maximum likelihood estimate are the methods of choice; the Kaplan-Meier estimator is the preferred survival analysis approach.