There continues to be a marked growth in the use of twin study methodology to address problems in the genetic epidemiology of common disorders. Examples include studies of cardiovascular risk factors, anxiety and depression, addictive behaviors, vulnerability to stress, and disorders of childhood and of aging. These are large-scale projects supported, internally and externally, by various Research Institutes of the Public Health Service. The growing investment in data collection has generated a special national need for investigators to become skilled in the most powerful and efficient methods of data analysis and experimental design. These methods include linear structural equation modeling employing maximum likelihood estimation for univariate and multivariate genetic analyses, analyses of longitudinal, time series or developmental data, resolution of causal relationships between phenotypes, and analyses of experimental power. To meet this need an intensive computer-based workshop on the methodology of twin studies has been developed. From 1991, in response to suggestions from participants, the workshop is being extended from one to two weeks to provide additional opportunities for trainees to consolidate their skills, to analyze their own data sets and to consider some advance topics. The first workshop resulted in the publication of a special edition of the journal Behavior Genetics on the genetic analysis of twin data and a monograph/textbook is currently in preparation based on the 1990 and 1991 workshops.