A Consent form is a signed document that confirms that a volunteer, “healthy” or sick, agrees to participate in a specific clinical study. In order to make the consent valid, the person who signs the document must do it in a free and informed way. It means they are not compelled to sign it (in any way), and must have completely understood what is going to happen during the study they take part in, and how it is conducted. Thus, before the exam, a doctor systematically delivers complete information on the investigation purposes, on the way it is conducted, and he answers all the questions that may have the future subject. For some investigations (called non-interventional), consent is not required. Then we talk about a non-opposition, i.e, a tacit agreement.  It means that the subject who participates in a given investigation agrees that their data are used, for instance. However, they can make an opposition to their doctor.