In medicine, differential diagnosis (sometimes abbreviated DDx, ddx, or ΔΔ) is the systematic method healthcare providers use to identify the disease causing a patient's symptoms. In other words, part of diagnosis is attempting to narrow down the list of possible diagnoses until one emerges as the best.

Differential Diagnosis is the process whereby a given condition or circumstance, called the presenting problem or chief complaint, is examined in terms of underlying causal factors and concurrent phenomena as discerned by appropriate disciplinary perspectives and according to several theoretical paradigms or frames of reference, and compared to known categories of pathology or exceptionality. Differential diagnosis allows the physician to:

1. more clearly understand the condition or circumstance
2. assess reasonable prognosis
3. plan treatment or intervention for the condition or circumstance
4. enable the patient and the family to integrate the condition or circumstance into their lives, until the condition or circumstance may be ameliorated, if possible.

If the patient's condition does not improve, the diagnosis must be reassessed. The method of differential diagnosis was first suggested for use in the diagnosis of mental disorders by Emil Kraepelin. It is more systematic than the old-fashioned method of diagnosis by gestalt (impression).

The term differential diagnosis also refers simply to a list of the most common causes of a given symptom, or a list of disorders similar to a given disorder.

The term differential diagnosis also refers to such lists when they are annotated with advice on how to narrow down the list (the book French's differential diagnosis is an example). Thus, a differential diagnosis in this sense is medical information specially organized to aid in diagnosis.