Center for Evidence Based Medicine, University of Lisbon: Medical practice should be based on high quality clinical evidence. Clinical trials are the gold standard for efficacy and safety of therapeutic and preventive interventions. A significant percentage of clinical trials fail to detect any benefit of the interventions under study – the so called negative studies. These tend to be published in a smaller percentage than positive studies (publication bias). For the health practitioner, however, it is almost as important to know “what works” as it is “what does not work”, so the results of all clinical trials (positive or negative) should be published, so their results should be available to the health community. This is why our Center for EBM at the University of Lisbon School of Medicine strongly supports the AllTrials campaign.