The concept of complexity is described increasingly in the medical literature and refers to the care needs of patients who have multimorbid conditions and the organizational structure of health care systems. This article provides an overview of the literature on this concept and discusses the need to reconcile case and care complexity. Case complexity has been operationalized in several ways. Conversely, the operationalization of care complexity has drawn much less attention. As an example, an empiric model to describe the interrelations of several indicators of care complexity is presented