IR-tree models assume that categorical item responses can best be explained by multiple response processes. In the present article, guidelines are provided for the development and interpretation of IR-tree models. In more detail, the relationship between a tree diagram, the model equations, and the analysis on the basis of pseudo-items is described. Moreover, it is shown that IR-tree models do not allow conclusions about the sequential order of the processes, and that mistakes in the model specification can have serious consequences. Furthermore, multiple-group IR-tree models are presented as a novel extension of IR-tree models to data from heterogeneous units. This allows, for example, to investigate differences across countries or organizations with respect to core parameters of the IR-tree model. Finally, an empirical example on organizational commitment and response styles is presented.