The most important statistical methods currently used for the analysis of twin data are described. The main objective of these methods is to estimate the contribution of the genetic and environmental factors to the variability of normal or pathological human traits, by means of the information obtained from monozygotic and dizygotic twin pairs. In this context, the concept of heritability becomes relevant. Not only the simple comparison between monozygotic and dizygotic twins, based on measures such as the concordance and the correlation, but also new and more complex approaches are presented, with a special emphasis on the structural equation models, and a synthetic view on the DF-analysis and the correlated frailty models. Some examples of applications to the data of the Italian Twin Registry are also illustrated.