Diagnosing multiple sclerosis (MS) is not always straightforward. The very nature of this disease means that it can manifest in multiple varied clinical presentations. Diagnosis is further confounded by the lack of a single diagnostic test and a gargantuan list of differential diagnoses. Traditionally the diagnosis of MS is a clinical one: with the history and/or examination revealing the separation in time and space of neurological episodes. In some cases, however, a diagnosis of "possible MS" might have to be made, for example, when the patient presents after a single event. Paraclinical investigations and diagnostic criteria may be of help in these situations. It is recommended that any diagnostic uncertainty is discussed openly with patients.