This work examines mental disorder as a circumstance that excludes from criminalresponsibility, seeking to explain why it is so difficult to establish unimputabilitydue to mental disorder, who is responsible for making this decision, and how theprinciple in dubio pro reo and the burden of proof are at work here. Thus, in fivechapters, the study reconstructs the historical evolution of dominant methods for itsdetermination in criminal process, explains the mixed method as the most accepteddoctrine at present, describes procedures and criteria recognized by the psychiatriccommunity to have scientific value, and, finally, introduces some elements takenfrom the theory of communicative action and systems theory to address how todetermine unimputability from outside the law and psychiatry. Thus, based on theseelements, the work reconstructs the process of decision making on unimputabilitydue to mental disorder and explains how the legal and health systems are related,through an analysis of the minimum conditions required for a good procedurefocused on this aspect.