Multimorbidity is defined as the coexistence of two or more chronic diseases. However, this complex health status, which primarily affects elderly, is still insufficiently understood. One reason is the underrepresentation of older, multimorbid people in studies. Another reason is that there is no agreement on the number and type of diseases, which have to be considered in the assessment of multimorbidity. Therefore, this article provides an overview on the status quo of research on multimorbidity indices and describes in detail, what kind of methodological challenges have to be faced regarding the development of a standardized index. Finally, recommendations are made for criteria, which can be used for the selection of diseases relevant for multimorbidity.