The genetics of asthma and atopy has been studied frequently in twin populations from various parts of the world. However, emphasis has been put on univariate analysis of questionnaire data, whereas clinical and intermediate traits only sporadically have been studied, especially in multivariate settings. This review focuses on multivariate twin studies of atopy and related traits. We conclude that the genetic liability to most atopic traits is significantly correlated but that trait-specific genes also play a role. Previous studies have estimated the genetic correlation between upper and lower respiratory allergic symptoms, that is, asthma and hay fever, to be between .47 and .95. Furthermore, atopic traits share a portion of their genetic determinants with other complex disorders like obesity and behavioral traits. A correlation of about .3 and .34 has been reported between genes associated with asthma and obesity, and between genes associated with asthma and depression, respectively. We emphasize that multivariate methods applied to twin studies, especially when genetic marker information is available, provide a valuable framework within which complex etiological mechanisms underlying atopy can be disentangled.