Curr Allergy Asthma Rep (2012) 12:590–598 agent or agents are responsible for symptoms, a liability claim usually requires expert opinions about the source of that agent. Where did it come from? Who was responsible? Historically, patients rarely ask: “whose pollen, which latex product, or where did the mold come from?” That changed for allergists when various environmentally-encountered agents acquired the dual role of both potential allergens and the subject of a lawsuit. This placed a new demand on allergists who were now asked to assist a patient by offering expert medical opinions about the strength of association or link between the suspected source of the allergen and the medical symptoms or illness observed. Moreover, the allergist must be prepared to explain the basis for that attribution in part relying on other experts who may have identified how, when, and why the suspected environmental conditions or agents occurred. But the aller- gist must be able to comfortably link those findings to the clinical disorder. He may be asked to support that causal nexus with these types of questions: “Doctor, given the airborne level of mold spores found and the description of the water damage, was the duration and extent of potential exposure sufficient to have caused the patient’s asthma?” “What is your basis for that conclusion?” The purpose of this discussion is to elucidate, for aller- gists, this interface between clinical responsibilities of the doctor and legal demands of the patient as claimant and the court system. Even for clinical allergists who have no inten- tion of or desire to be formally hired as experts, causal questions involving both the documentation of the specific allergy and the assessment of the source of the allergen(s) are important to the patient who is also a claimant. For an allergist hired by the plaintiff or the defendant’s attorney as an expert witness for the purpose of establishing causation, both questions, their answers, and a critical analysis of the bases for those answers are the linchpins of the expert’s role. In the discussion to follow, we shall attempt to delineate those questions which must be asked and answered by the allergist as expert witness. We will begin with the medical causation issues, followed by the source identification issues. We will then present the legal requirements and show how and why physicians can be led astray or have their testimony excluded by judges for lack of a proper method- ology or foundation. The goal of this article is to provide the allergist with the necessary information to assure reliability and admissibility of his testimony so it is helpful to the court and to minimize the risk of a disconcerting experience. Medical Causation Analysis The science of causation assessment is well accepted and generally recognized. This is the field which refers to the scientific methods by which, for example, a microbiological 591 agent, chemical, allergen, irritant, drug, or medical device can be causally connected to or ruled out as a cause of disease. To determine whether a causal relationship exists be- tween a given agent or substance and adverse health effects, it is essential that proper causation methodology be applied. Methods of assessing causal relationships are not matters of opinion. Rather, methods for investigating causal relation- ships between agents and illnesses have been well delineat- ed in the scientific literature [1–14]. One simplified overview of the components of a proper causal analysis was published by one of the authors who called the methodology the “Does, Can, Did” approach [6]. “Does” refers to the diagnosis: from what ailment(s) does the patient suffer? “Can” refers to the capability of the alleged causal agent to actually cause the identified disor- der(s). And “Did” refers to the result of specific investiga- tion carried out to link the putative causal agent(s) to the specific individual patient. The first component, “does,” is the diagnostic element. This, of course, is familiar to the clinician because it is the bedrock of clinical as well as medical legal practice. However, the very fact of a claim against an employer or a presumed tortfeasor (a person who has committed a wrongful act) adds a confounder to the diagnostic encounter which can lead the practitioner astray if not considered. The next two elements: “Can” and “Did” have, since the introduction of these descriptive terms, been named by the courts as “general” and “specific” causation. Numerous decisions concerning expert testimony and the very admissibility of an expert’s opinion have turned on these causal elements. General Causation In order to arrive at cause and effect conclusions regard- ing exposures and health effects or disease processes, certain elements are crucial. First, there must be scientif- ic knowledge that the agent is capable of causing the health effects or conditions in question. This element has been called “general causation.” Second, there must be sufficient information available to conclude that the agent was causal in this specific individual. This is called “specific causation.” In many specialty areas, general causation is established through epidemiologic studies. The method by which those studies are combined and weighed to assess causal proba- bilities has been most closely connected to a speech by Sir Bradford Hill in 1965 [10]. Dr. Hill was speaking about the growing numbers of studies which linked lung cancers to cigarette smoking and asked: how many studies of what quality are necessary before we can assert a causal relation- ship? The principles which he laid out in that speech are commonly called (though not exactly correctly) “Hill’s