This paper describes the nature of professional diseases, their legal definition and the interpretations given by the courts in several cases. The study addressed causation, the negligence of the employer and the following liability beyond the application of the legal insurance, and describes some circumstances in which the employer cannot be blamed, such as the exclusive negligence of the victim, morbidity, time bar, and those situations in which the worker entered into the disease before is actual work, as an independent worker or being working to a former employer.