This is Part 1 of EM Cases' series on Diagnostic Decision Making with Walter Himmel, Chris Hicks and David Dushenski discussing the intersection of evidence-based medicine, cognitive bias and systems issues to effect our diagnostic decision making in Emergency Medicine. In this episode we first discuss 5 strategies to help you master evidence-based diagnostic decision making to minimize diagnostic error, avoid over-testing and improve patient care including: 1. The incorporation of patients' values and clinical expertise into evidence-based decisions 2. Critically appraising diagnostic studies 3. Understanding that diagnostic tests are not perfect 4. Using the concept of test threshold to guide work-ups 5. Understanding that the predictive value of a test depends on the prevalence of disease We then go on to review some of the factors that play into the clinician’s and patient’s risk tolerance in a given clinical encounter, how this plays into shared decision making and the need to adjust our risk tolerance in critical situations. Finally, we present some strategies to prevent over-testing while improving patient care, patient flow and ethical practice.