In the daily clinical practice, doctors often use their experienced “gut feeling” and/or “spot eye” in the assessment of their complex patients. According to this point of view someone could affirm that the “true clinicians” do not need any formal tool to define the complexity of their patients. But this statement is not always true. Indeed the concept of complexity in Internal Medicine lacks a precise definition. “The Complexity of Measuring Clinical Complexity” is the title of an editorial published on the Annals of Internal Medicine in 2011 [1].