In this EM Ed lecture we discuss the mindset that is needed when approaching an undifferentiated patient. This is the mindset that is used in the emergency department given that most patients seen in the ED are undifferentiated when they hit the door. It is important to start with the default mindset that all patients in the emergency department is critical. You can always deescalate from this position after you evaluate the patient. Understand that you have limited data when determining whether your patient is sick or not sick. You are a detective trying to figure out what is going on. Rule out badness first.