While twin studies have been used to estimate the heritability of different traits and disorders since the beginning of the twentieth century, statistical developments over the past 20 years and more extensive and systematic data collection have greatly expanded the scope of twin studies. This chapter reviews selected possibilities of twin study designs to address specific hypotheses regarding the role of both genetic and environmental factors in the development of traits and diseases. In addition to modelling latent genetic influences, current models permit inclusion of information on specific genetic variants, measured environmental factors and their interactive effects. Examples from studies of anthropometric traits are used to illustrate such approaches.