This chapter describes how the heritability of a trait can be estimated using data collected from pairs of twins. The principles of the classical twin design are described, followed by the assumptions and possible extensions of the design. In the second part of this chapter, two example scripts are presented and described, explaining the basic steps for estimating heritability using the statistical program OpenMx. OpenMx and the scripts used for this chapter can be downloaded so that readers can adapt and use the scripts for their own purposes.