Background: Most episodes of anaphylaxis are managed in emergency medical settings, where the cardinal signs and symptoms often differ from those observed in the allergy clinic. Data suggest that low recognition of anaphylaxis in the emergency setting may relate to inaccurate coding and lack of a standard, practical definition. Objective: Develop a simple, consistent definition of anaphylaxis for emergency medicine providers, supported by clinically relevant consensus statements. Discussion: Definitions of anaphylaxis and criteria for diagnosis from current anaphylaxis guidelines were reviewed with regard to their utilization in emergency medical settings. The agreed-upon working definition is: Anaphylaxis is a serious reaction causing a combination of characteristic findings, and which is rapid in onset and may cause death. It is usually due to an allergic reaction but can be non-allergic. The definition is supported by Consensus Statements, each with referenced discussion. For a positive outcome, quick diagnosis and treatment of anaphylaxis are critical. However, even in the emergency setting, the patient may not present with life-threatening symptoms. Because mild initial symptoms can quickly progress to a severe, even fatal, reaction, the first-line treatment for any anaphylaxis episode�regardless of severity�is intramuscular injection of epinephrine into the anterolateral thigh; delaying its administration increases the potential for morbidity and mortality. When a reaction appears as �possible anaphylaxis,� it is generally better to err on the side of caution and administer epinephrine. Conclusion: We believe that this working definition and the supporting Consensus Statements are a first step to better management of anaphylaxis in the emergency medical setting.