Testlet effects can be taken into account by incorporating specific dimensions in addition to the general dimension into the item response theory model. Three such multidimensional models are described: the bi-factor model, the testlet model, and a second-order model. It is shown how the second-order model is formally equivalent to the testlet model. In turn, both models are constrained bi-factor models. Therefore, the efficient full maximum likelihood estimation method that has been established for the bi-factor model can be modified to estimate the parameters of the two other models. An application on a testlet-based international English assessment indicated that the bi-factor model was the preferred model for this particular data set.