Cohort studies are observational studies that follow a selected group of patients, called a cohort, for a period of time to observe medical histories and treatments that occur to these individuals over the review period. Principal characteristics of these types of studies are to have repeated observations or reviews of the patients over time, serial evaluations for the occurrence of diseases or diagnoses, and performance of physiological procedures on these patients to determine a phenotypic characterization of each individual. These phenotypes can then be used to examine genetic influences on the overall incidents and prognoses of diseases. C-TASC staff have carried out numerous cohort studies in the past 20 years. Most recently, a study was completed which establishes national standards for bone mineral density during children’s growth years from 6 to 20. Other cohort studies conducted by C-TASC include The Women and Infants Transmission Study (WITS), which sought to identify the risk factors associated with vertical transmission of HIV from pregnant mothers to unborn infants, and to investigate how pregnancy affected those women. This was an exciting study that demonstrated the remarkable success of the HIV medical community in reducing medical transmissions from 25% to less than 0.5% with the onset of highly active anti-retroviral treatment protection regimens administered to the mothers over pregnancy. Analytical techniques frequently involved in this type of study include survival analysis and longitudinal data analysis, including mixed-modeled analysis of variance and generalized estimating equations, all of which are routinely employed by the C-TASC Statisticians in these and other types of studies.